I have no idea where his experience comes from. I've mentioned this on HN before, but my 5 year old Galaxy S3 has only gotten faster since launch and the battery lasts longer; with its current 7000mAh extended battery lasting 10-15 days without needing to recharge, and no that's not a typo.

I swapped the Samsung ROM to CyanogenMod (now LineageOS) shortly after release; inmediately gaining some speed from the process and noticeably improving with each major Android version bump.