I've also been getting this freeze for almost a year now! First I thought it was my MacBook Pro, that it / or the video card in it might be dying. Then, I thought it was High Sierra being buggy. I contacted Apple and have searched around the internet high and low, and found nothing, until I came across this thread here after searching on "stalling for iohidsystem". Thank God I found it.
The freezing was driving me absolutely crazy, and has frankly been getting worse, and I didn't think it could be my beloved BetterTouchTool. Until I saw this thread.
I would try to check the Mac's Console app when the freeze occurred (I would leave it open all day just to catch it), and found errors similar like these:
fault 16:49:33.902047 -0700 kernel virtual IOReturn IOAccelEventMachine2::waitForStamp(int32_t, stamp_t, stamp_t *): initial wait for 1 second expired. Continue wait for 4 seconds. stamp 502913 (gpu_stamp=502912)
fault 16:49:37.902351 -0700 kernel virtual IOReturn IOAccelEventMachine2::waitForStamp(int32_t, stamp_t, stamp_t *): timeout waiting for IntelAccelerator stamp 502913 (gpu_stamp=502912)
fault 16:49:37.902365 -0700 kernel void IOAccelEventMachine2::handleFinishChannelRestart(IOReturn, int32_t, uint32_t) - Setting restart type to 4
fault 16:49:37.902370 -0700 kernel void IOAccelEventMachine2::handleFinishChannelRestart(IOReturn, int32_t, uint32_t): GPURestartDequeued stampIdx=4 type=4 fromWhere=1 waitingOnIdx=4 default 16:49:37.902373 -0700 kernel [IGPU] display event timeout on index = 4
fault 16:49:37.902426 -0700 kernel void IOAccelEventMachine2::restart_channel(): GPURestartSkipped stampIdx=4 type=4
fault 16:49:37.902429 -0700 kernel void IOAccelEventMachine2::restart_channel(): no channel associated with stamp_idx 4 (type 4)
I used my Mac for a few days without BTT running, and I got zero freezes, when I would otherwise get 3-10 per day, some so severe and frequent I could barely move my cursor and had to reboot.
Btw, there are lots of threads on Apple's support communities about this freezing, so you might want to post these solutions there to let people know what is going on and that it isn't their Mac or High Sierra (like me, before I found this thread). I counted about 8,000+ people reporting this issue (who used the "I also have this problem" button) across about 6-8 threads on the Apple support forum.
Anyway, I do love BTT and your hard work on it, so thank you. It has become an essential app for me on my Mac! I look forward to a fix soon, which I have no doubt you will be able to provide, and congratulations to you and your wife on getting married! All the best, and thank you again!
(btw I'm on v4.427 (787) of BTT and v10.13.6 of Mac OS / High Sierra)