I use my macbook 16 with an external display (lg 4k 32'') in clamshell mode.
It was fine until I updated from 10.15.4 to 10.15.5. Everything looks very laggy: cursor, window animations, video, everything.
Whenever I open the lid and use both monitors, it comes back to normal. But whenever I try to go back to clamshell mode, it lags.
My other workstation has the same monitor but it's still in 10.15.4, so I'm not updating that machine until this bug gets fixed!
I was reading some forums and it looks like an older version of macOS had a bug similar to this one. Another user claimed that he/she could go back using Time Machine, but as I use another software to only backup my work files, I can't test it out.
Anyway, in short, if you have a setup like mine DO NOT update to 10.15.5!
When I saw the new beta 2 update from apple today it gave me some hope (I didn't have beta 1 installed). But after work I updated to the new beta and realised it doesn't fix this issue.
I don't understand how apple can roll a public version with a bug like this, when (I'm just assuming) many developers probably use Macbooks hooked to external monitors at work and at the end of the day take the computer to go.
I'm not even considering reinstalling the whole OS, just in case someone suggests that. It's not a major issue to use the macbook open next to the external monitor. It's a incovenience, sure, but not worth reinstaling the whole system just to try if it works.
I also tried connecting the display over different ports and different cables. Same result. As it was right after the update, I'm 99% sure that's the cause.
Hope my long text help someone out there.
Best,
Pedro.
