I'm using Debian testing too, and noticed the delay. Before, KDE Plasma was moving quite in timely manner in Debian. By now released version is already 5.5 and 5.6 approaching, and Debian still is stuck on the older one. There were some serious Qt bugs[1] which were causing a lot of instability which were fixed in Qt 5.6 so I thought this delay was related.
[1]: like this one: https://bugreports.qt.io/browse/QTBUG-42985