VIII’s 32-bit I believe.
In the Steam client install folder if you right click and go into the properties menu for GameOverlayRenderer.dll
Security
Advanced
Add
Select a Principal
Enter your account user name and choose that
Switch from allow to deny
Show advanced permissions
Uncheck everything but “traverse folder / execute file”
Ok / Apply / OK (Confirm on apply that you want to do this.)
It’s a bit lengthy but this is done to unload the 32-bit Steam overlay .dll from loading for testing.
Normally you can just rename the file but Kaldaien mentioned on the Steam community forums how a more recent change to the clients behavior is to keep this loaded into memory so I’m not sure it’s that simple anymore.
If performance is no longer affected it can be at least tracked down a Steam overlay issue going by your last post on how non-Steam games work fine.
In SpecialK setting Silent=True for the config file and not using any Steam functionality could also work for the other end of this issue assuming it’s the overlay that is.
EDIT: Oh and then edit and delete the added security permission or delete the file and let Steam validate and add a clean one so you don’t forcibly stop the overlay for all 32-bit programs. 
Unfortunately there’s no easy way to just skip the overlay for one game and the client toggle just hides it from drawing you still get the GameOverlay.log file and activity, it’s just invisible.
EDIT: And then the larger issue is resolving the actual problem without losing functionality, overlay has some uses tied to it both for Steam itself and SpecialK’s usage for it.
Confirms (hopefully) where the issues coming from but yeah resolving it is a different matter.
There’s a few settings in SpecialK but I don’t think they’d help here.
(Loading some stuff early or attempting to load a Steam API file and functionality among some other settings.)
EDIT: And I’m mixing multiple problems here, first there’s the crash problem then there’s SpecialK and somehow resolving a performance issue with the game?