Switching to tab with comb boxes with many items is very slow

I have an application where, upon a file being loaded into it, a set of combo boxes are generated dynamically with it. Each combo box has roughly 3000 items (all backed by the same QStringListModel). These combo boxes are on a separate tab. Opening that tab after adding the combo boxes is very slow. About 3-4s in a release build.

Only adding all of the combo boxes in the first place already takes about 1 whole second.

This slow loading only happens the first time around a tab is opened however. Once I have opened that tab at least once, switching around between them is instant again.

I attached a video displaying this behavior as well as a small reproducible sample that displays the issue on a smaller scale.