-k
does not slow anything down, but it needs 4 file handles for each job. If GNU Parallel runs out of file handles, it will wait until one of the running jobs finishes.
-g
compared to -u
slows down around 1-2 milliseconds per job (plus the time it takes to write and read back the output from disk), so the slow down will only be noticeable if you run very short jobs or jobs with much output.
--line-buffer
can be faster and can be slower than -g
. It does not buffer on disk, but it takes more CPU time to run - especially if your jobs output data slowly.
My recommendation would be to use what is easiest for you to use, and only if that proves to be too slow, look into the other options.