After Troubleshooting and updating a brand new iMac running BigSur 11.2.3. iMac would perform fine and then user would report sluggish performance specifically using adobe photoshop and indesign, during shutdown the bigsur image would hang and present with a divided color tone straight down the middle. Profile was originally migrated from an older iMac using the migration assistance. Tried creating a new profile and moving the files there which worked for a little bit, but the problem reoccured. As a final effort I reformatted, performed an SMC reset and NVRAM reset and handed it back to the user. He said it was working perfectly for the morning and then it began running slow again. The only difference was he connected to his synology NAS, as soon as he ejected it everything was working fine again. I hope this helps some of you. I'm not sure if its all externals or just network shares but that seemed to be the constant for us. At all times after set up the user either had the NAS connected or a usb external drive. Try ejecting any drives you may have or performing an SMC and NVRAM reset, (I'll note too Apple support instructed me to simply boot into safe mode, sign in and restart. I'm not sure the mechanics of this but they stated it does fix a number of odd problems for some reason).