Hey, I've made buffer fix for HTC U12+ in a form of modded lib (needs manual replacement):
It will fix lagging viewfinder in GCAM both nightsight and normal mode.
Boot into TWRP, mount vendor and system, replace file manually using TWRP file manager. (system/vendor/lib/hw)
Tested on custom android 10 rom
If you want, you can support my work by sending me donation
XDA: DevDB Information
HTC U12+ TWRP replacable lib fix.