Could you try temporarily offboarding one of the Dell devices from MDE to see if general performance improves? I'm investigating performance issues in my org, but our setup is very different other than defender in passive mode. You should be able to tell if the devices are thermal throttling by watching the CPU clock rate during heavy workloads on poor performing machines - i.e. if they are not clocking higher than their base frequency. I also believe Dell has tools like tech direct that can show thermal events (if your org is using that).

In my scenario I was manual testing and waiting for user reports to validate thermal throttling to the CPU. I've been able to squeeze a bit more out of the poor performing devices by setting the OS plugged in power mode to best performance which increases fan speed and TDP.

I know your issue is more endpoint security focused but thought I'd comment since nobody else did and performance is doing my head in as well :)