If fsync on one file forces all other files to disk, then it is 100% an OS performance problem.

If your temp files fit in ram along with running programs, then they should stay cached. A ramdisk should only be needed in edge cases where you want to manually force other things out of memory, or because you particularly want to avoid extra writes to disk. In general performance is supposed to be boosted just as much by caching.

If your disk head is writing the temp file, then it can't be sitting idle waiting to fsync. And it'll eventually write as part of checkpointing.