Sketch and Flow save each stroke and item as a separate files in RAM. After some time the allocated memory fills up and it starts to slow down. The very easy fix for this is to save your progress, close the program in question, and then reopen it. If you have multiple instances open of the program close down each one and then open them back up. The more RAM you have the longer it will take for something like this to happen.
Next check to see if there are any program that are running updates in the background. More often then not with Windows 10 system it is updating at the time this issue occurs.