I just checked mirrortube, its UI is indeed intuitive and easier. Part of it is probably due to its support for a single interval repetition whereas ours is a multiple interval one. Still it offers some insights on UI, so thank you. Also, can you elaborate on "not nearly as smoothly"? (I believe that's related to the core functionality, and not UI. Do you see a lag in seeking back to the start time in every loop, for example?)
Thank you!

EDIT: Please check the new version, where buttons 'Start' & 'End' are provided. This should simplify the step of entering timestamps.