I used Kraken's API quite intensively for several months (until about 1-2 months ago), and in the last times it became virtually unusable (errors, no replies, delays,...). However, I tested it some days ago and it improved quite a lot, becoming usable again.
On Bittrex I traded only manually, but only from time to time: no problems so far.
From my experience (about 1 year) on cryptoexchanges, the usability tends to worsen dramatically during large market movements, most probably due to overload.