The Hurd is currently slower than Linux, yes. But not very much, so it is completely usable.

Take care when running the Hurd in fully-virtualized machines: virtualization software use ugly heuristics to make Linux run faster, which will not work on the Hurd (or BSD, etc.) so comparisons in virtualized environments do not really hold.

The main reason for slowness is not because of the overhead of RPCs. It's mostly simply because less care has been done on implementing what makes Linux fast: intelligent read-ahead, carefully-tuned page cache, etc. or even just missing DMA support for your disk controller.

There is no ground reason this can not be achieved on GNU/Hurd, it has just not been a priority until now (first make it work, then make it work fast). We are currently working on multi-page pager and read-ahead, which should improve this a lot.