Roblox can basicly run on the most trash phone ever, and still be at least 15-30 fps (10 on best graphic settings).
Btw, it almost complety depends on the game you play on roblox.
If you play a game with like 3 Objekts, the fps will never go below 60.
If you play a game with like 2 billion objekts, the fps will (unsuprisingly) never go higher than like 5 fps (also, dont blame my english, i am not actually english)