Interesting.
I just tried reproing (ST4 1937 on iPad 9th Gen iOS26.0.1, 137 songs in song list) and could not.
I tried with Top Panel Display set to Waveform and Audio Region Controls.
I tried lying flat on table vs in Portrait mode (a few quirks reported recently based on orientation).
I had a timestamped lyric sheet with highlighting and audio regions playing while opening/closing the sidebar (both on Song list and Playlist modes).
I can definitely see CPU load jumping to low 80s (baseline is low 20s on my iPad without messing with sidebar) as I open/close the Song List. The way the song list currently renders on open - it always Opens at the top of list and then visibly scrolls down to the selected song - seems to incur quite a CPU cost.
Is it possibly something that only occurs over time? I.e. reboot iPad, launch only ST4, play song, open/close sidebar - does that perform better and then does performance/behaviour worsen as you play more songs (i.e. over time)?