weeeeellll... as you know I view things a little differently... "if it ain't broke don't fix it" philosophy. If something is weird figure out what it is before going apeballs on EVERYTHING and possibly breaking a bunch of other crap. Nothing else has been broken on this config since X3 so I wasn't about to go futzing around... a) because there was no reason to and b) I have more interesting things to do when I'm in front of the DAW.
This restart lag is most definitely a new thing with SPlat. I honestly don't think that this driver update is going to fix now would spending a bunch of time trying to isolate every possible thing on the rig that could possibly be out of date (and most of the "updates" are going to solely be for Win8 or 10 compatibility improvements anyway).
However since it's my audio interface (obviously a crucial component) and it's coming from Focusrite (trustworthy, lots of users on 7 here who would have reported issues, etc) then it is something I'd do whether this problem was occurring or not. If it fixes it, great.
It's a lot easier to pin down where problems are coming from when you aren't constantly cramming stuff on to the system just because it's the latest and greatest. That's how I can be pretty sure this is a Sonar thing. The only other variation made on the system is the fact I installed/updated my graphics card driver/software shortly after installing SPlat but if that's what's doing it then it's an argument against updating all willy nilly (but I did install that for a very specific reason and it serves it's purpose).
This also, as I said, seems to be tied to the SIZE and complexity of a project. My small projects or new/empty projects don't take as long to release. My medium projects take a little longer. This mammoth project takes the longest.