To serve a single client, it doesn't matter, but when you're serving more than a few thousand of clients concurrently, there's benefits to a faster web-server. I mean, more concurrent clients on the same hardware, better average/worst-case latency, need for less hardware, less chance of a bottleneck...
That still doesn't make much sense. The gains of sticking with a proven, solid server which you are familiar with surely outweigh the 0.5% speed increase.