AHCI increases read speeds vs ide, usually highest percentage gain is small read speeds that OS typically uses, (large percentage gains .5 to 8Kb in atto). Write speeds can also be affected depending on particular drive/firmware, pic below is mine booted up in ide vs ahci, and why I use ahci mode.

Boot up time is same if using new ahci rom, which does not pause for ~7-8 seconds while searching for drives. If not using a modded bios with new ahci rom, then boot up time is annoyingly 7-8 seconds longer.

ahci vs ide on mine both run today.