Default StartupNotify time-out is too long in Xfce
The default StartupNotify time-out value seems to be set to 30 seconds in Xfce. That's too long and some applications like Firefox might make the cursor spin for 60 seconds despite no CPU activity. There are many applications were StartupNotify don't work properly so the cursor won't stop spinning until time-out is reached.
I suggest making this value configurable by the user/admin (I think it is in KDE) and it could be a hidden setting by default. If that's not possible I suggest setting it to the same default value as in MATE which is 15 seconds.
I started a thread on the Xfce forum with some more info: https://forum.xfce.org/viewtopic.php?id=14617
I'm surprised there aren't more complaints about this, but I guess most Xfce and Gnome applications work correctly with StartupNotify so if a user sticks to those the experience won't be bad related to StartupNotify. However, there will always be many non-functional (related to StartupNotify) applications (I made a short list in the link above) that require a "better" value than 30 seconds. Otherwise these applications will give a bad impression of Xfce to the casual user.