I think another thing that may trip people is that many programs simply display a splash screen when first loading, then the program GUI pops up as it loads the project.

SONAR pops open the GUI even as the program first loads (making the delay more obvious). A splash screen may make things less "noticeable." Adobe even takes this to streaming all of the DLLs on the splash... They go by fast, but the programs actually can be slow to load as well. It just gives the feel of "speed" to watch DLLs blur by.

JamieVJohnson
I work with CAD software and this sounds so familiar, since i'm also a Platinum user (with formerly installed X3) I can at least offer a different point of view:

1. Programs are slow because MS Windows is slow, particularly when timing out searching for a dead search path.  CHECK ALL THE FOLDER MAPPINGS IN THE FOLDER LIST, and make sure each one is correct and to the proper version, an upgrade program may have copied an old path to a new program causing a mismatch.
2. Drivers, DRIVERS, and more of the same.  Two programs were written on two versions of MS .NET programming, plus each one came with it's own set of hardware/software drivers.  You could simply be loading a driver in the new program that was intended for the old program.  Then the program gets into a loop on a simple routine such as load track, and that loop fails over and over and over again, causing the error handler to pick up and fix the problem over and over and over again, and whala, you have time wasted loading a project.
3. The Project Files themselves.  You have audio data, midi data, video data, links to plug-ins, and controllers, and all the like.  If S3 and SP map them out differently, then hopefully only the first time I open a massive project will I see a lag, the second time should be way faster as it is 'upgraded' into the newer system.  Try this Open the slow project, SAVE AS new project (such as Bogie-S3 vs Bogie-SPlt), just to make sure the next time you reopen its using the upgraded project file.  Upgrades may be subtle like adding a 1 or 0 based list in the system, but see item 2 for how a subtle problem can add up quickly.

In the end result, I only have installed the 1 version I use, and all is well.  So buy a new machine, you wanted to upgrade anyway, or install Microsoft Hyper-V and create a new OS install of windows (perfectly legal).  Run one with the old S3 and one with the new Splt, and test things out in separate playgrounds until you have found the exact culprit, then post back here with your findings.
 
jvj

Thanks for the tip
 
This prompted me to check my folder locations in preferences which I knew were fine. I found out however Platinum had reset the project folder path back to the default. I shall check this again on the next update.

You don't know what's going

alewgro
i am using the same PC, same OS install, and I will confirm that Sonar Plat is loading the same projects slower.  Should this be submitted to the bakers?

You don't know what's going on... Maybe Sonar has a new in built validator/parser to fix corrupted projects for instance. Or maybe it's just going around in circles for no reason. Who knows..