I originally was using filament I could print very slowly (25-30mm/s) to get high quality output. I found that many filaments did not print below 45mm/s. I found that the printing problem was acceleration control not speed. I went to the options on the LCD display and set the accelerations to 400/500/500. This was a pretty big drop in acceleration but allowed me to print at 70mm/sec. I could now use any filament and print at a speed of 70mm/s with high quality finish. My prints take about 20% longer but the output is worth it.