Which Unity version are you using?
I just tested with Unity 2020.1 with the demo project (i.e. also with animation events) and even with a 5 FPS limit the footsteps played correctly.
I'm not really aware that there are issues with animation events going missing on low FPS. If that's the case, there's not much I can do about it in Footstepper, as that's a Unity thing. You could try using auto play or using footstep triggers (see more in the playing footsteps documetation).