Since the size of the non-pre-allocated hash space doubles (aproximately) every time it has to expand, the number of reallocations is relatively small compared to the number of hash inserts you are doing and has a negligable effect on your hash performance. The aproximately 10 to 12 reallocations that the hash had to go through to accomodate 10,000 elements is a small chunk of time compared with the 10,000 hash inserts. As shown in your benchmarks, the pre-allocated hash is faster over the data set, but only slightly so.