slow scanning speed bug still exists on linux

This old issue (#2381) is still affecting installs today. I'm new to retroarch, but this "fix" vastly improved scanning speeds for me. I'm currently scanning atari 2600 ROM directories. Its taking ~18 hours and still had 10% left. But when I mirrored the steps in this comment, it immediately sped up significantly.

After following this process the scanning takes under an hour. Thats like a 20x improvement. This is NOT trivial, so I'd appreciate someone looking into it.

          Found this:
          - start scanning a big directory, let say all SNES roms
          - launch a NES rom
          - hit space

          Scanning speed unlocked.

Originally posted by @kivutar in https://github.com/libretro/RetroArch/issues/2381#issuecomment-158003634

Can someone please investigate if this is a bug in the default configuration? I do have "gamemode" installed - which is enabled in retroarch.

Version 1.16.0 (89a67e5)

Thats all the info I think is relevant to the issue.

Start by reproducing the Linux scanning slowdown with a large ROM directory using the reported version and compare it with launching a NES ROM and pressing space during the scan. Trace the scanning workflow and default configuration to identify why that action changes the speed; done means scanning proceeds at the normal speed without the workaround.