OP this is caused by low framerate. When I play on my laptop my framerate sometimes drops and when it gets particularly low my movement speed slows to a crawl, even with auto-run on.

Things like this SHOULD be server-side (as they are with most MMOs) to ensure that movement speed is a static value but instead here if your computer can't process fast enough to keep up then it gets slow in calculating your position too... and the server doesn't think it's strange in the slightest that your character is moving at a speed outside any of the defined norms (hence why we have so many movement/teleporting hacks).