Shifting between package header and package body (either using Ctrl-H or clicking on the tabs) is slow in version 15.
In version 14, it happened almost in an instance, but in version 15 it is more sluggish (like a full second), you can litterally see PLD removing all visual objects and code and then redrawing everything again.
When you have a package with only a header (e.g. for types or constants), the sluggishness stands out when you press Ctrl-H, because here there is nothing to be shifted as I only have one tab page.

Could seem that it has (also) something to do with the icon size. I have chosen the smallest-but-one icon size. If I change it back to largest icons, it performs better.