[SOLUTION] {
1. Save As... with "Copy all audio with project" and "Create one file per clip" checked,
OR
2. Wait. It appears that X3 needed time to process all the clips in the session (1800+). After a...while...the session ran like it was supposed to. And in fact gets going just a wee bit faster than X2 (X2 seems to open a default screenset then changes to the one I last had, which takes a moment; X3 goes right to the appropriate screen configuration)
1. didn't take as long to do as 2., but with 2. I get the full clip that I can still edit if I hosed it up, so: yay.
}/ end solution...I now return you to your original post...
 
I am getting start/stop latency with X3- lots of it- that I am not getting with the same file in X2.
Project contains a 15GB avi (1hr 15min  and 22s) file, 52 mono tracks, but no more than 12 playing at any one time, plus the original stereo audio track of the avi file.
I originally had 8 Sonitus compressors, and 2 instances of Prochannel's EQ, but all FX have been deleted and FX disabled.
 
1. When i press play, the video and sound playback  start, but the Now Time/ Now Time Marker nor any level meters work (this is intermittent as sometimes the Now Time and level meters work).
2. The Hard drive in running continuously.
3. When I press stop, there is a 22 second delay before playback stops. Sometimes. Sometimes it stops immediately
4. There is a further 16 second delay before I can do anything with X3, as the "I'm doing something" mouse icon is displayed...because apparently X3 is doing something. And that's IF X3 understands that it's supposed to stop. There is ALWAYS this minimum 16 second delay after playback stops. Sometimes the "I'm thinking" icon goes on for longer.
5. The "thinking" icon pops up randomly even when the transport is idle.
6. This behavior happens regardless of whether the thumbnails are displayed or not.
7. This behavior happens after deleting the video, saving, and restarting X3.
8. The session works fine in X2, even after opening the X3-saved file in X2. 
 
X3 isn't crashing or white-screening, but apparently it can't handle this project like X2 can: it's the same file with the same plug-ins.
 
On the plus side: my music sessions seem to be working fine in X3.
 
Win7 x64, 12GB ram, 64 bit X3a and X2a, NVidia GeForce 9400GT, i7 920 @ 2.67GhZ

Do you know what codec the video is using? Also, do you have video thumbnails turned on, and do the problems go away when you turn thumbnails off?

That's a pretty big file for a 1:15 avi - I'm betting it's either MPG-2 1440x1080, or MPG4 1920x1080....
 
Normally in such situations, I would make a low-res proxy avi in order not to tax the CPU. Final Audio can easily be mated back in a video editor.

That's a pretty big file for a 1:15 avi - I'm betting it's either MPG-2 1440x1080, or MPG4 1920x1080....
 
Normally in such situations, I would make a low-res proxy avi in order not to tax the CPU. Final Audio can easily be mated back in a video editor.

It's a 720x480, 29 frames/second, 48kHz stereo .AVI, rendered via Adobe Premier Pro (other settings unknown at this time)
Problems persist with or without thumbnails 
The problem persists after deleting the video entirely 
The problems persist after archiving the video's audio.
I could possibly decrease the video resolution but I'm doing ADR and Foley and that defeats the purpose.
 
I'll stick with X2 for this project, I suppose, as X2 is working just great. But I could really use the comping and track lane features of X3.

It may not make a difference but there is a recommendation for X3 upgrade to reset and rescan plug-ins http://www.cakewalk.com/S...reader.aspx/2007013315
 

I had a large project that was having trouble too until: I checked the preferences for sync and caching and enabled the disk read caching option and upped the cache size to 1024.
You have to click the advanced button at the bottom of the preferences window to see the sync and caching options.
 
I remembered I had this enabled on x2 but for some reason these settings did not migrate even though I told the installer to migrate my settings. Seems all that was migrated were the driver settings.
 
Hope this helps.
 

Well, if you deleted the video then that's not the problem, unless it's being buffered in memory while the project is still open. I would try deleting the video, then doing a Save As under a different project name, then close and reopen Sonar and load the project without the video. If you still have problems with that project, then you know for sure that the video isn't what's causing the problem.

Tom Riggs
I had a large project that was having trouble too until: I checked the preferences for sync and caching and enabled the disk read caching option and upped the cache size to 1024.
You have to click the advanced button at the bottom of the preferences window to see the sync and caching options.

+1 tom, these settings do not get migrated.  i had the same issue from x1>>>x2.  52 audio tracks i would recommend 2048.  

Thanks for the suggestions!
Here is what I have discovered so far:
- I checked my settings on X2 vs X3: everything migrated properly, and X2 doesn't use the disk caching. But I enabled it. But the disk caching didn't help.
- I archived all of the audio files...that didn't help..that's weird.
- I then saved the session to a new directory, copying all audio with the project and creating one file per clip. Doing this solved the problems. I also walked away while this was happening to make a bit of food...but I don't think eating directly effected the computer's performance at all.
 
So my hypothesis is that each clip within SONAR X3 has some data associated with it due to the new comping features or something and by saving each clip as a file, it enabled X3 to fix all of the slip edits while saving, where-as the original file just needed time to go through all of them and I wasn't giving it that time (1800+ clips after the save...more in the audio file of the original); kind of like PT rendering the waveforms and fades, etc, in the background. 
 
So thank you all for our help!