This has to do with monomorphic versus polymorphic and megamorphic functions. Basically, the inline cache has finite capacity, and if every call of a given function takes objects of the same shape (they share a hidden class), then you don't need to worry about evicting your cache entries.
Once you start passing in objects of different shapes though, you're going to exceed the inline cache's capacity, and start losing out on the massive speedups the cache gains you.
A function that takes one inline cache entry is monomorphic, more than one is polymorphic, and more than the inline cache capacity is megamorphic. You want as many functions as possible to be monomorphic, polymorphic if you can't help it, and never megamorphic.
Once you start passing in objects of different shapes though, you're going to exceed the inline cache's capacity, and start losing out on the massive speedups the cache gains you.
A function that takes one inline cache entry is monomorphic, more than one is polymorphic, and more than the inline cache capacity is megamorphic. You want as many functions as possible to be monomorphic, polymorphic if you can't help it, and never megamorphic.
This post gets into a lot more detail: http://mrale.ph/blog/2015/01/11/whats-up-with-monomorphism.h...