The amount of lag different things like mushroom spread cause is difficult to determine. Theoretically it is possible but it has, to my knowledge, never been done.
As such it is difficult to give specific guidance on what configurations will cause lag and which wont.
As a basic guide, just keep it small. I don't think I can define to the block what small is, but use common sense.
If you really want lots of mushrooms, just grow some big ones with bonemeal and harvest.