When I asked this question, I make the distinction between "currently used optimization techniques and algorithms vs their implementations in GCC/clang". How large is the gap? If the gap is small, it means the algorithms as currently used for optimizations are inherently expensive (i.e. a "good" compiler "must" be slow). if the gap is large, it means a much faster implementation is theoretically possible (i.e. a "good" compiler "can" be fast).
LLVM does indeed use known suboptimal algorithms for increased speed. Its register allocator is an example: a "perfect" register allocator is NP complete, and LLVM uses a relatively simple greedy algorithm instead.
