Kdenlive 18.08.3 had instantaneous horizontal timeline zooming (in/out) using Ctrl+Scroll, while it is very laggy on 19.08.2

It is the exact same project.
It is a long project (>20 minutes), yet the older Kdenlive version could zoom in and out immediately, without lag. (using Ctrl+Scroll).

Also a side question: Why was the track height adjustment shortcut removed in a recent KDEnlive version? (it was there in 18.08.3, but no longer in 19.08.2)