The problem here is rclone delete is using recursive listing by default and this essentially means that it does each HEAD request single threaded. You can disable this with --disable ListR.
According to my experiments with a normal directory structure (generated with rclone test makefiles --files 10000 --max-file-size 10b 10000files) the speed of listing with an age filter seems to max out at about --checkers 16 with --disable ListR.
What you are doing
$ time rclone delete --min-age 100d TestS3MinioManual:test --checkers 32
real 0m10.957s
user 0m4.254s
sys 0m0.760s
Now with --disable ListR
$ time rclone delete --min-age 100d TestS3MinioManual:test --checkers 32 --disable ListR
real 0m1.588s
user 0m5.489s
sys 0m0.732s
So I think you should be able to get a significant speedup by adding --disable ListR to your rclone delete.
You can also try this with just plain rclone delete (no --disable ListR)
v1.63.0-beta.6959.b87a9c835.fix-listr-performance on branch fix-listr-performance (uploaded in 15-30 mins)
This does the HEAD requests in parallel so is a lot faster. However it still only fetches one page of things from AWS at once so will top out at 1024 HEADs in parallell.