Summary
The rate at which an SSD can process TRIM appears to be vendor specific. I wrote a script to measure the impact of a large amount of TRIM on read IO performance. I tested it on two servers I have at home that are described here as v4 (SER4) and v6 (socket2)
Results from iostat for the Samsung 870 Evo are here. The read IOPs are ~110k /s before and after the TRIM. They drop to ~105k /S for the 1-second interval in which the TRim is done.