By default, Kustom updates music info every 5 seconds, which causes that stuttering or skipping effect.
If you want the progress bar to update smoothly every second on your home screen, do this:Open Kustom Settings - General Settings.Go to Advanced Options - Update Mode.Set Update Mode to Fast. Keep in mind: this mode uses more battery.
Also make sure to set Kustom to Unrestricted in your phone’s battery optimization settings.