Today I tried to write my first song in Qtractor today using MIDI and samples in the arrangement. At 145 BPM I used approximately 3 MIDI items and 15 audio items per bar. It played well on my fast desktop until I copy and pasted the song to 32 bars long, then it turns into a slideshow when I scroll the arrangement. During playback I only use 10% CPU, but when I try to scroll or zoom, one CPU core goes to 60% and stays there until scrolling finishes.
I think the problem may be the same as one of these.