When dragging the scrollbar the thumb is not updated immediately on Windows, which makes scrolling feel more sluggish as it really is. The content updates quicker than the scrollbar thumb itself.
This happens with and without OpenGL but with OpenGL enabled it is much more notable.
Tomaž Vajngerl committed a patch related to this issue.
It has been pushed to "master":
tdf#99574 fix sluggish scrollbar for an immediate paint
It will be available in 5.2.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
fix for 5-1 -> https://gerrit.libreoffice.org/#/c/24912/