They can't do a load before actually reaching the load instruction, and they can't fill the instruction buffer all that fast, maybe 5 instructions per clock, de

They can't do a load before actually reaching the load instruction, and they can't fill the instruction buffer all that fast, maybe 5 instructions per clock, depending on loads of factors. They don't know what instructions are on the critical path, so it is perfectly possible to pick wrong ones in this regard. Having a big instruction buffer does not necessarily mean that the CPU can always pick a perfect selection of instruction from it, that is already a very hard issue that only becomes harder when you give it over a hundred to pick from. In short, the out of order instruction buffer can do some amazing stuff for code that would otherwise run much slower, that doesn't mean you can't gain or lose performance by reordering instructions. For non-trivial code the best composition is almost certainly different between CPUs.