I also have the 2019 MacBook Pro and it's been a dumpster fire. I'm running 3x 4K monitors and it's completely unusable with the dedicated GPU (the 5500M).

I spent months trying everything I could think of: downgrading to Catalina, turning off transparency/shadows, running as few background services as possible, and not using scaling at all (which was the most effective solution). And this was only with 2x 4K monitors; I added a 3rd more recently.

Nothing worked. Thermal throttling and insufficient sustained power were two problems I was able to identify (the 96W adapter is not sufficient for the system's peak power load, so it uses the battery to get over 96W of draw).

Eventually, I broke down and bought an eGPU (Blackmagic eGPU) which solved the problem. For about ~$700, I'm now able to use my machine without a hiccup. Not a great or affordable solution, but it has made my $3,100 machine usable again.

I have an i9 2019 MBP 16” at work and also don’t notice a slow down when I bring it home. Maybe it’s a GPU bug?

I’m pretty sure my i9 model has the lower end 5300M.

Another question: what are you using to connect the monitor to your laptop? USB-C to DisplayPort, here. I formerly used HDMI off of a USB-C hub but it was a bummer.

Myself and my cofounder can reproduce the exact same issue every time with each of our MBP 2019 5500’s. Another team member with a 5400 running the exact same dev environment as me has never had the issue, so there’s something funky on that model.

It’s pretty bad the Apple still denies any issue, not being able to use an external monitor at all through covid suuuuucked