OK, for all of you that have reported these problems, here's another thing to look for.

In Vegas, use the Explorer built into Vegas to find one of the files that you have put on the timeline, and which is playing slowly. Right-click on that file and then select "Properties." Scroll to the bottom of that dialog to the section that is labeled "Plug-In." Now, if the "Name" is listed as "m2tsplug.dll" and the "Format" is listed as "Sony M2TS," then your video has been captured correctly. However, if instead the "Name" is listed as "mcplug.dll" and the "Format" as "MainConcept MPEG-2" then your video has not been captured correctly and as a result is being played back within Vegas using the generic MainConcept code. The preview -- and everything else about your edit session -- is going to be slow, and there is nothing you can do about it other than try to recapture and hope everything goes right.

Now this is only with Vegas 7. If you are trying to play back m2t (HDV) files in Vegas 6 or earlier, then the playback is ALWAYS done with the MainConcept MPEG-2 plugin, and the playback will always be slow.

As to why this happens, I don't have a clue.

Thanks for rapid and informative answers
but - quote 1
"Now, if the "Name" is listed as "m2tsplug.dll" and the "Format" is listed as "Sony M2TS," then your video has been captured correctly".
Does the format Sony M2TS mean that Vegas 7 only capture correctly from Sonycams?
Any Canon HV 20 owner who knows?

Quote 2
"........and there is nothing you can do about it other than try to recapture and hope everything goes right."
Does this mean it depends one good luck for correct capture in vegas 7.

Thanks again - this forum is great!
/Aje

Does the format Sony M2TS mean that Vegas 7 only capture correctly from Sonycams?

No, it definitely does NOT mean that. The format designation has to do with the fact that the decoder was written by Sony (the Madison software group) and not licensed from MainConcept. One of the biggest strategic errors ever made by the Sony software group was continuing to license MPEG-2 technology from MainConcept rather than developing their own. As you know, they made the decision, very early on, to develop their own DV AVI codec, and the results in rendering quality and playback performance are fundamental to why so many of us were attracted to Vegas in the first place.

By contrast, they never developed their own MPEG-2 codec, despite its fundamental and essential importance to DVD and HDV. However, they are finally putting their little toe in the water in Vegas 7 by dealing with SOME, but not all, variations of MPEG-2 using their own technology. However, if they would take the complete plunge, then we could have lossless cutting of m2t and MPEG-2 files, faster playback for ALL MPEG-2 files, better handling of VOB files, faster and better MPEG-2 rendering for DVD and m2t print to tape, and much more.

Does this mean it depends one good luck for correct capture in Vegas 7.

No, not on luck, but on correct configuration.

BTW, you never answered the question in my post: What does the Properties dialog say for YOUR video?

Sorry!
File name: Clip 001.m2t (no .dll)
Format: MPEG-2 (as you said with Vegas 6)

If I got you right I could benefit an upgrade to Vegas 7.
Thanks again Johnmeyer for your superrapid information.
/Aje

If I got you right I could benefit an upgrade to Vegas 7.

Yeah, big time.

Basically, if you edit HDV, then Vegas 7 is nice to have. However, if you want to edit m2t files directly, then Vegas 7 is essential. No downside to the upgrade (other than $$) that I've ever found.

Hmmm... very interesting and correct. I captured 5 (HDV) clips from my HV20 in Vegas 7d (now using e). The clips were actually captured in one session but Vegas used scene detection to create (correctly) separate clips where I had started/stopped my camera. Preivew/playback on one of the five clips (#4) sucks wind. The others playback very smoothly. Checking the properties of the clip in the Explorer window I see JM is correct - the one clip with the sucky playback is using the Mainconcept rather than Sony MPEG-2 plugin. But how can this be anything other than a bug in Vegas given the clips were all captured together in the same session using the same settings? Don't know if ver. 7e fixed this, but clearly an issue.

Here's another one related to previews/playback - how come the exact same project (24p DV) on the exact same machine playsback/preivews smoothly in Vegas 5 and with the same preview settings, chokes in Vegas 7?

Have now installed Vegas 7e.
Captured from HV20 and opened m2t file properties
in Vegas explorer - it says:
Name: Clip 001.m2t
Format is not mentioned but
Type: MPEG-2 Transport Stream
Why isnt format presented in the file properties?

Anyhow what a difference in previewperformance - much better.

You need to check the properties of the clips in the Vegas "Explorer" tab, not the "Project Media" tab to see which plugin is used.