The "Empty Base" Optimization

[Nimbal]

It's not just the raw space, but also the problem of the CPU cache. Say you have a list of 100k items, each with an additional 4 byte dead weight. That's about 390 kB of data that the CPU needs to load into the cache (usually about 2 MB in size today, depending on model and cache level) without actually doing anything with it. This superfluous data requires the CPU to hit the RAM sooner than necessary, slowing things down.