COPY the .dlls to the location where most of your VST plugins already reside. The key here is to consolidate and then remove duplicate locations from the list that Platinum scans. For example, I took the .dlls from the Native Instrument plugs and copied them in the folder that contains most of my other VSTs and then removed the Native Instruments folder from the folders/ directories  list that Sonar uses to scan for VSTs. You may not have to do this if you only have one or two sources, but admittedly, I had a lot of directories where VSTs  were residing. (3rd party plugs installing their own directories, Band in a Box directories, Sonar directories, etc) You're not actually moving the plugins, just copying and consolidating them. I would leave the "originals "where they are ,so your other music related programs can find them, just remove them from the list that Sonar looks at on startup. that way it won't be looking at the same VSTs twice.

The VST scan introduces a lot of background overhead and is really unnecessary unless you have installed something. I set X1, X3 and Platinum to "Manual Scan" and X1 is about 2 seconds, whereas X3/Platinum are both under 3 seconds (cold start conditions after a reboot from SSD). Also bear in mind that X1 is a significantly "less complex" program than either X3 or Platinum.
 
Edit: I just set X2 to manual scan, and it uses the same scan as X1. Another thing to bear in mind with this differentiation is that X1/X2 do not have ARA or VST3 support... Never really occurred to me until I went to open a Platinum project in X1 and plugs were "not found" (they were all VST3). The "toast" VST scanner also has "VST3 Migration" options to 1) Hide related VST2 plugins and 2) Replace VST2 plugins when opening projects. Both of these (if enabled) affect both boot and project load times (which does not exist in X1/X2).

Platinum probably takes a little longer to start because of the new licensing routine.

Starting with X2a, SONAR uses newer Windows libraries to add Multitouch support. This had the side effect of dropping XP support.

I have tried the suggestions here (reduce number of VST folders from 2 to one, pinned Sonar to control bar), and nothing has made a difference. Again, X1 starts up instantly, X2 and newer take a minute. If I understand the comments here, multitouch and VST3 support were added in X2. Maybe this is what is causing the additional startup time.

After Dorchester, my startup time for Platinum went from 30 sec to 3 sec (both on SSD). I have disabled startup scan, and do manually scan, when I have installed something new (which I sometimes forget, and get a bit irritated, because I cannot find the new toy I know I have installed).

Pre-dorchester there was an issue where the VST scanner was sending a lot of messages to SONAR slowing it down on load. However that issue was addressed in dorchester. If SONAR appears sluggish while a VST scan is happening then its definitely this problem. If you see this in Dorchester, try deleting your vstscan.exe file from the shared utilities folder and running just the SONAR installer again. That should restore the latest VSTscan exe. Let me know if that fixes your issue.

By the way. Did you upgrade to Dorchester?