It will be anywhere between 20 and 150. Usually 20, Does this mean something is keeping minecraft from running at it's full potential or is it just a coincidence? I've allocated as much ram as possible.
Computer specs:
Amd8 6410 APU With raedon5 graphics
Windows 8.1
6GB ram
Quadcore
Try changing your view distance (the number of chunks loaded in front of you).
Or maybe your mods are the ones to blame.
If its stock then the drop in fps means the game is rendering more chunks… Which normally take a lot of processing power… Allocating extra ram won't help unless ram is the only thing slowing it down… Instead your gpu and or your cpu is what is slowing it down.
What exactly is "radeon5 graphics"? If you mean Radeon R5, then you are probably luck to get 20fps. It's a low end cheap video card that does not perform well. Plus you are using Windows 8.1, another performance hit.
Get a decent video card, and either use Linux or Windows 7, and you will get good performance in Minecraft.