that is entirely within reason, can you try some simple 64x64 texture on them, see how much FPS that drops, alternatively, COULD a shader be faster instead of using skins? I don't know enough about shaders to make that call, but it might be worth knowing from a shader expert.

edit: also in minecraft, textures aren't filtered and shading is disabled and instead calculated externally from light sources, day of time, etc, and then lightens or brightens the blocks accordingly, so that could also save some render power. Try disabling texture filtering and default shading, see how much that would help the fps