I am having slower performance on Debian 11 with software only from the "main" repositories after upgrade from 10.It is on a decently powerful Dell laptop with 16 GB memory, 1 TB SSD, 7 processors Intel Core i5-8350U CPU @ 1.70GHz
How much is slowdown? If it's in the range of 10-15%, that's
expected given the fixes for Spectre/Meltdown. There are knobs to
turn them off (search for 'make linux fast again'), but they make
you machine insecure.