I haven't found it too slow, but there are clearly load issues. Sometimes you search and get no results, when the same search produced 82 results last time. Also when you go to next page it can lose all your results and have to start again with the search.
I've found that its best to set returned results to 100 per page and then click open in new tab so you don't lose all your returned results.
There are also occasional matches against wrong documents. All of this is indicative of a load issue (when using a caching API layer). I'm sure they will sort it out in time.