Yes! Aetherium Client is entirely free to download and use. There are cosmetic items available for purchase, however, no gameplay feature is limited or hidden behind a paywall.
Yes! In most cases with optimal settings Aetherium Client will double your frames per second compared to vanilla Minecraft!
Aetherium Client currently only support 1.8.9.
Yes! Since Aetherium Client use LegacyFabric as a mod loader. You can find all the mods available here.