"is practically nothing on modern hardware" if the data is already present in the L2 cache. Random RAM access that stalls execution is expensive.

My guess is this is why he didn't see any speedup: all the code could fit inside the L2 cache, so he did not have to pay for RAM access for the deference.

The number of different classes is important, not the number of objects as they have the same small number of vtable pointers.

It might be different for large codebases like Chrome and Firefox.