Performance with overlapping UVs

I’ve noticed when dealing with newly or badly unwrapped models that my computer performance really slows to a crawl when handling overlapping UVs in the UV editor.
It’s fairly easy to get out of the situation by trash-unwrapping the offending geometry, so I don’t expect it to be high on any fix-list… but it can still be pretty annoying. I’m mainly curious if anyone knows why, if it happens to be understandable by a non-coder at all.

On my computer, a “overlap-unwrapped” cube of 16x16x6 (=4096) faces lags like crazy… just to make clear I’m not talking about sculpting level poly counts. And Pack Island clears it up, so it’s not the number of islands.

In this case I encountered it when playing with this wireframe-rendering method, which relies on 100% overlapping UVs.
To give an example of “bad” UVs used for a functional reason.