Same problem here - 2015 MBP with i7, 16GB, dual graphics card. - but for now solved. I posted a couple of days ago about how i was about to downgrade my machine - on unplugging the external monitor everything looked to improve. Well, i've been able to go further and two things were definetly affecting the machine. I've changed from using the internal HDMI port to a mini-display adapter - i can now get my external monitor reconnected and it's isn't taking 100% of the CPU I've reverted back to allowing the os to control the fans rather than 3rd part app - and after a short burst from the fans to drop the temp down from about a 65degrees to 50, its' gone back to running fine. I would be worth checking you are not having thermal issues (high temps) forcing OSX to increase it's reliance on push up kernel_task CPU usages which looks to block user doing much with the computer until it's reduce the temperature back down.

Same problem here - 2015 MBP with i7, 16GB, dual graphics card. I was about to downgrade back to an earlier OS as was becoming unusable - 90-100% CPU within minutes of booting. I've tried all the suggestions (SMC, updates, disable any 32-bit apps etc) but nothing worked. Just as I've moved my machine to the side whilst it backs up before a downgrade, I've disconnected the external monitor (an ultra widescreen 34" Samsung). I come back to it after a few minutes and CPU is almost zero - system nice and fast and responsive and looks to be just fine. Arrgh!! Too early to tell whether the complete fix but I would be interested to know whether people see a similar result once they disconnect any external monitors. I'm thinking in this case whether bigsur is having trouble constantly switch between the two graphics cards rather than can't support the larger monitor (2gb graphics card)