Enabling certain plugins takes a long time, once and again on every subsequent start of MuseScore
Enabling certain plugins in Plugin Manager, in this case Batch Convert, takes ages, some 35 seconds.
Another plugin, Color Voices, doesn't suffer from this (but doesn't work in the Windows version of MuseScore 3.0.2, even if it did in the pre 3.0.2 development versions, see #283105: builtin plugins don't work)
The builtin plugin "ABC Import" also takes that long to get enabled.
The builtin "Note Names" plugin does not (but doesn't work either, see above).
The builtin "Color Notes" plugin also takes ages to load (but doesn't work, see above)
To add insult to injury, not only the first enabling of that plugin takes ages, also every startup of MuseScore gets delayed by about the same amount, a normal startup, like on double clicking a score, takes 3-4 seconds. With one of those plugins enabled it takes 35-37 seconds. with both being enabled it takes more than 70 seconds!
This is on an i7, 8 cores (4+HT), Windows 7
Workaround is to not use plugins at all :-(