Are you running the steam version?
I am having a similar problem, after a while alt+tabbing out of construct 2, checking steam, opening the animation editor in construct will start to slowly slow down the editor a lot, till its at the point you need to quit it and reload ( using beta 117 )
My guess is something isnt quite right with the steam intigration, my graphics drivers are up to date, pc is up to date etc etc
I use Steam version and I'm experiencing the same, I think you're right. Tried on 3 PC's, drivers up-to-date, RAM from 4 ddr2 to 6 GB ddr3, graphics RAM from from 1 to 2 GB ddr3 to 5, 32 to 64 bit architecture, computer and registry quite clean or very few applications running at the same time if not only system services.
Wherever I looked for info, the only ones blaming have the Steam version. It seems quite a cyclic phenomenon, apparently not dealing with autosaving or backing up the project (but I cannot tell for sure, perhaps with some configuration set it happens fewer times). C2 becomes slower and slower or gets temporarily stuck (especially - but not only - when the prog window is open and you haven't worked on it for some time), and that sometimes forced me to restart the program (or both the program and the Steam client).
It is to say: C2 does not shorten the hardware resources that much (except for Ram) and if you reduce the program to icon one or several times, its normal speed may get restored. Saving the project doesn't improve the situation but seems to slightly help. If there wasn't this lag, not many times I had issues with this fantastic piece of software. Anyway, hope it can be useful.