Because you take a huge hit in IPC. Somewhat less if you make your binaries extremely CPU specific. For instance even something simple like the load instructi

Because you take a huge hit in IPC. Somewhat less if you make your binaries extremely CPU specific. For instance even something simple like the load instruction. To answer that you have to know the state of ALL levels of cache, their size, associativity, contents, TLB, etc. Otherwise it might take 0.3ns or 100ns. To be able to do nothing (because you are in order) is a huge bottleneck.