I recreated your scenario on my iPad (App Store version 1.5.6.39), and didn't run into a freezing problem. My farmer jumped off her horse each time she encountered a slime, slayed the slime, and then jumped back on her horse. Slayed maybe 6 slime and was able to exit the Secret Woods with no problems.
Which SDV version are you running, and is it the App Store or the Arcade version?