Thank you, OldSmoke - but I was not seeking an answer - just trying to inform other users here that the GPU "accelerated" rendering in some special cases gives render times up to 10 (ten!) times as long as not "accelerated" render times and even induces clear stuttering errors into the rendered media too.
In my daily work GPU assist improves some few percents with my cheap GTS 450 card - as expected - but as it improves the preview speed I do not want to switch it off permanently.

Also I thought that this very significant example - which I have seen several times before - might interest SCS and give a hint as where to seek and then improve the Vegas software.

I do, of course, use the 296.10 driver. Most later drivers seem to be kinda April 1. jokes from Nvidia :- ))

As to my hardware I do have a 1 kW power supply, real ECC RAM and 2 real Xeon processors.

I do understand that you want to point this out but I still think you have to look into the hardware first. Your two Xeon might certainly be faster then your GTS450. Do you have a sample veg file or can you tell me what kind of 15sec material you are rendering with which template? I just rendered a 1920x1080MXF 35VBR file to AVC MP4 1080p in 27sec. with GPU on but that might not be same as you have done.

Vegas needs to update gpu support. It used to work very well. It is sony's responsibility that it works & they have dropped the ball big time. Sony as whole is failing in many ways.

Anything else muddies the water, confuses the matter, & shifts the responsibility.

@Lovelight

It still does work very well. I use it since it was introduced in VP11 and never had an issue with it. Even VP12 works extremely well for me, build 394 that is, and now that I have 2x GTX570 I am extremly happy with GPU acceleration.

Oldsmoke - yes, sure, it is correct that a GTS 450 normally only improves rendering speed a little - compared to dual Xeons or other fast CPUs.
That is what one might expect and it has been well known in this forum for a long time.

But my statement was and is that when I render i.e.a Sony FX Starburst then the GTS 450 does not even improve speed at all - it decreases rendering speed by a factor of 10 (ten) - from 1 minute without GPU to 10 minutes with GPU - and on top of that GPU assist induces quite significant stuttering in the rendered media.

This is a fact on my machine when the GPU assist is on - and it was that fact that I wanted to share so that other users experiencing the same problem might save time.

If you want to try, btw. - here is the veg (Vegas 11) with media:
https://www.dropbox.com/sh/1yna2wcnvvbkfe1/6GYXYfLOjP

My rendering settings are: mpeg-2, 1920x1080, 25, upper field first, VBR at 31,28,20 Mbps.
You may not be able to play this smoothly on a (normal) PC but also a lower bit rate will do - as Norman writes below.

"You also must select CUDA in the render template, don't leave it in automatic or
OpenCL."

One thing people need to not overlook is that there are two "renders" going on.

One which always happens. Vegas is rendering the video stream from the various sources, corrections and effects.

The other is when we render a file of some kind.

Each has their own GPU settings and neither care about what the other is set to.

Vegas (video stream render) is always OpenCL and is controlled via Video Prefs. As for a file render, that is specific to the encoder in question and is controlled via the encoder options if it has any GPU support.

In the example given it was talked about how Starburst stuttered on GPU but was okay with CPU in their machine setup. Vegas is doing this computation generating the video stream and it is irrelevant what render template you use.

@ritsmer

Thank you for providing the sample files. I opened it in VP12, Build 394 and rendered it our using the mpeg2 bluray template and modified the bit rate accordingly.

GPU On: 1:09
GPU Off: 4:30

Note that the mpeg2 templates have no option to select which way to render, GPU or CPU but I did change that in the preference/video tab and restarted Vegas for the test.
This is the GPU On result: https://dl.dropbox.com/u/39278380/GVK-LowThirds09-GPU-ON.m2v

The CPU render test is a different story; it seems that one or more of the desired FX didn't get rendered at all: https://dl.dropbox.com/u/39278380/GVK-LowThirds09a-CPU.m2v
I guess my 3930 just isnt as good as 2 Xeon.
I also rendered to MC AVC using the 1080p template and that took 34sec with GPU on, I haven't rendered with CPU only but I gues it would take at least twice as long.

I will do the same in VP11 since I have 10,11 and 12 on my system.

I appologize if I miss understood your initial post. All I am trying to say is, that it is not so much a software but hardware related issue.

Edit:
VP 11 Build 701

GPU On: 7:55
GPU Off: 7:58

The mpeg2 encoder in VP11 doesnt use any or very little of the GPU hence to lower and close to CPU render time. However, the result looks somewhat different from VP12; go figure. https://dl.dropbox.com/u/39278380/GVK-LowThirds09a-VP11-GPU.m2v

I have the feeling that one or some of the FX isnt GPU accelerated in VP11 at all. I switched of track 2 and GPU usage is quite high, up to 75% and render time is very short. In comparison, the GPU usage in VP12 was up to 93% with all tracks enabled.

I wonder how good and fast your system would be if you can get VP12 working.

Oldsmoke: Also thanks to you for trying.

Quite astonishing that your systems V11 build 701 render-times are GPU:CPU 1:1 while my system shows 10:1.
Could be because I use build 700 (32 bit).
In your GPU ON rendering result there is, however, precisely the same stuttering as I see here with GPU ON.

I have tried to render in my V12 build 486:

GPU ON renders until 14% and then obviously hangs.
Noticed some strange kinda interlaced-looking-lines in the lower third of the preview window.
Tried press Cancel and Yes to stop background process - but had to kill V12 in the Task Manager after some time.

GPU OFF: rendered to the end and slightly faster as V11.
Noticed in both the preview window and in the rendered outpout the same significant difference between this V12 and my V11 as you have seen.
I have checked this and it seems that the Brightness and Contrast FX in V12 removes most of the Starburst "corona" around the curved green line when I click it ON.

I am really thankful that you took time to test this issue.
Think that we have proved, that in the present example:
1) GPU ON induces quite noticeable stuttering in the rendered result.
2) V12 Brightness and Contrast FX works astonishing different in V11 and V12.

Glad it works well.

My point is how many times has nvidia updated its drivers compared to Vegas updates. How could Vegas stay ahead of the game when the updates are far & few between? How can they even keep up with nvidia? I have other gpu enabled software that is much better than Vegas at rendering.

If it vegas gpu works so well than why is all this testing being done?

These are not easy questions to answer because of practical, psychological, & financial issues.

Glad it works well.

Hmmm - in my opinion Oldsmoke and I have identified 2 repeatable serious issues... :- )

ritsmer, have you tried the earlier build of VP12? I find 394 to be more stable on my system and I don't have the "red preview window" issue.

I personally prefer VP12 over 11 as it utilizes my GPU better and is faster in both, playback and render. The flickering in the VP12 render you noticed is actually on the timeline too. When play it back, I can see the green animated line behind the text to make sudden changes, even when I go thru it frame by frame.

OldSmoke: well - I have been enjoying Vegas since it was called Screenblast 3.0 and have been quite active for years and for many Vegas versions trying out this and that in order to "optimize (whatever that might mean :- ).

I have, however, now reached a state where the editing itself is more urgent to me that experimenting with number of threads, preview RAM, Nvidia driver versions, hacking DLLs etc etc.

So - like many others around here - I have found a setup that simply works rock stable in my hardware environment: Vegas 11.0 build 700 (32 bit) - and I will stick to that for the time being.

Remembering the great improvements to speed for AVCHD that SCS introduced in version 10 (10b or c I think?) and several other good improvements I continue dutifully to buy the newest versions and try them out - when I have time.
Unfortunately the first versions of V12 smashed even my faithful V11 (!!) so badly that I had to totally un-and-reinstall everything (do not have a ghost-backup) - so I am not so keen on further experiments with V12 before the next update.

For me the speed of the final rendering is not so important: speeding up all the very frequent small temporary renders to check the precise timing of something is more interesting - and so I consider changing to a PC with Intel 4000 graphics which seems to render astonishing fast in reduced quality - which is sufficient for these timing checks - and is what I use anyway (reduced quality) for this purpose.

I have ceased using GPU assist - and even if this makes the preview a little slower it means no more "surprises" in the rendered results - and you should really be able to trust your NLE and not have to scrutinize every final render more times to check for such - ah - little surprises.