Doing the disk defrag and the other scans is good.... also consider a registry scan/repair. Every time I do a registry scan it finds a number of issues. Good links to locations helps it find things faster.
That may or may not help.
It kinda sounds like it's taking the time to load lots of things from disk to memory. I would try an experiment.
Start a new project and record or drag in one audio track... it can be any audio.
Close X1 and then reopen it. Note the time. Now add one synth track and record a few measures. again repeat the close/re-open and note the time.
Your post #20 says that you believe it IS a VST causing the issue.
There may be one synth in particular that is the culprit. One synth that is taking 4 minutes to load. You may just need to find that one, and it may take some detective work to do.
Keep notes on what synths are in the project. Then, if and when you find a synth, that when added, causes a long load time, delete if from the synth rack so it's gone from the project and repeat the close/reopen.
add it again.... recheck the load time, and then remove the OTHER synths and recheck.... It may be one synth, it may be several..... but unless someone else has the definitive answer, this is how I would start on the quest to locate the reason for the long load times.
You can and should also do this if you have not yet done so. Cold boot the the computer and when you run Sonar, RIGHT CLICK the icon and select Run as Administrator. Don't ask me the particulars but doing that will sometimes allow it to run normally and do what it's supposed to do even when you simply run as a user from that time forward. It's worth a try. I know doing this has solved some other issues in the past for a few people.
If all else fails, an email to tech couldn't hurt.... just be patient on the reply.