I've been working towards getting more requests per second.
I'm doing this on not such a good machine so take this with a grain of salt (HP elite 190a). E.g. my hard drives are quite old and worn with noticeable speed issues. Even things like AV effect its performance. For this they are disabled.
To do this was basically commenting out at https://github.com/rejectedsoftware/vibe.d/blob/06adc36e73a588f4563d33396a95c3891b2bb373/source/vibe/http/server.d#L1431
I'm currently getting around 5.3k requests per second.
The other things I have done differently is that I have implemented a router that generates its checking code at compile time via delegates (because routes are known at CTFE).
There is around 18 routes currently being served of which only one (index) is being tested. It outputs basically just a HEAD request.
Strangely manually specifying a HEAD request with ab is slower by around 100ms.
For routes that doesn't exist, I'm getting around 5.2k requests/s. And with -i on ab around 4.2 which is quite a bit different. However my router handles 404's and so should not be causing an exception.
I rewrote my router specifically to test this out at. Before hand the most I could get was 1.3k. I don't know if its an accumulation of factors or simply regex harming.
I'm a lot happier with this now than what I was a few days ago.