Well.. I did an NVRAM restart (cmd+opt+P+R) - and it solved the issue! as well as re-setting the SMC on my 2015 MacBook Air.. Wow.. that fixed it all.. My MacBook Air has also been getting very very hot since updating to Big Sur.. will see if the above re-sets fixed that as well. Hope this helps all.

I have a 2016 MacBook air 16gig Ram, plenty of room on the SSD, and yet after the update yesterday 15th Dec, Xcode is running terrible. It has never been a rocket on my simple Macbook Air, but it's been very useable. However it is extremely slow now, to a point where it's very frustrating and dissapointing.