Don't quote me on this but I took a quick glance at the code and it looks like all gets are [currently] serialized at the vnode level. Since your key will always hash to the same vnodes (increasing machine count doesn't help) it means that request will pile up on the vnodes' mailbox.
I also see an asynchronous get defined in the vnode but don't see anyway to use it.
That said, if this isn't an artificial use case then I'd probably throw a short-lived cache in front of Riak for situations like this. I mean, if this were a more traditional database, you wouldn't run the same query 5000 times if you know the answer isn't likely to change.
