cat: improve throughput & simplify (split PR)

[oech3]

Split #12651 to make diff smaller. This part is needed for GnuTests.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout-group (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/basenc/bounded-memory is now passing!

[cakebaker]

Thanks!

[oech3]

Thankyou. rebase #12651 against this.