Last year I built an AM5 PC with x670e board and installed 2x gen4 ssds and 1 gen3 from different OEMs.
Today I installed another gen4 and ran benchmark on all drives. To my surprise all the old ssds read speed is down by 30-60% from last result, write speed is similar to specs. I am unable to figure out what is wrong with them all drives are showing 98-99% of spare.
Which os are you running?
Try to partition it with free space at the end and see if it makes a difference.
Try to trim the drive and see if it speeds up again.
Do you use any disk encryption?