That speed looks just fine. Remember your starter will probably be done very quickly. I did a two liter test batch with Denny's Favorite 50 and a 1.040 starter on a stirplate at 68 degrees. Tested at 12 hours and every two after that. It was at terminal gravity by the 18th hour. As part of the experiment I reached out to a microbiologist who works for 1 of the "big two" liquid yeast companies. I explained my intent was to refrigerate the starter to pour off most of the beer it made and pitch the slurry. Specifically I asked if the yeast needed to rest after hitting FG prior to refrigerating. The recommendation was to refrigerate as soon as possible after hitting terminal gravity. Keeping the yeast warm and especially using a stir plate post FG is not good for the yeast.
My current process is to shut off the stir plate around the 18 hour mark and refrigerate around 24 hours.