Stringer wrote on 4/20/2015, 7:01 PM
Before what?

Something has changed on your system though you may not be aware..
Any recent updates of anything on your system?
Do you allow ' automatic ' updates..

Do you have a hard disk that is near full?

Check your ' task manager ' and see what else is running in the background.
Dan Sherman wrote on 4/20/2015, 7:38 PM
Temp files topped up!
Also, recentlyydownloaded Comodo antivirus.
Told it wasn't a resource hog.
Maybe it is?
astar wrote on 4/21/2015, 4:10 PM
I run Win 8.1 and just rely on Windows Defender, with a periodic scan with Malware Bytes. AV software has always been a serious system overhead, that has gotten better with more CPUs and faster CPUs.

I was going to say check to make sure your drivers are all up to date, and an older driver was not automatically installed via windows update.

Verify the DCP latency on the machine, and correct any issues there.
johnmeyer wrote on 4/21/2015, 6:20 PM
If you don't mind, let me just copy/paste from a post I made in December. Before I do that, you should definitely re-boot. I have found that a lot of people almost never re-boot, and this can make a difference, especially with programs, like Vegas, that do a lousy job cleaning up after themselves. I recommend to not let ANY program (including Windows) do any automatic updating. This is pretty much guaranteed to have you wake up one morning to a computer that worked great yesterday, but now has problems. If something has been "upgraded" by one of these auto-upgrade background "helpers," you should be able to undo that by using System Restore, if it is enabled.Now for the advice I have given on previous occasions (this question has been asked before):

Four rules for fast preview speed

Also, if you search, you will find several dozen threads with additional advice. You may find various posts in these threads that will give you some clues on how to fix your problem.

Here are links to other threads where I gave this same advice. I am providing these in order to give you links to other threads where people were having the same, or similar problem:

Vegas Pro 10: 720p=ok, 1080p=Choke

vegas preview quality - still not getting it

Please critique my AVCHD-to-DVD production. :-)

Found One Cause of Slow Playback Speed

MultiCamera HELP - 6 Tracks of Jerky Video

OT: laptop speed