The extra checks added by the recompression of toast data introduced in
bbe0a81d is proving to have a performance impact on VACUUM or CLUSTER
even if no recompression is done.  This is more noticeable with more
toastable columns that contain non-NULL values.

Improvements could be done to make those extra checks less expensive,
but that's not material for 14 at this stage, and we are not sure either
if the code path of VACUUM FULL/CLUSTER is adapted for this job.

Per discussion with several people, including Andres Freund, Robert
Haas, Álvaro Herrera, Tom Lane and myself.

Discussion: https://postgr.es/m/20210527003144.xxqppojoiwurc2iz@alap3.anarazel.de