I think a user in our office found a bug within the Acrobat software. The user's position requires her to use the comment/markup tools all day and it wasn't until she got a new iMac that she started to have an issue with her tools. After bringing it to my attention, I was able to replicate the issue on two other different (older) machines running different versions of Acrobat.
ISSUE
A spinning beach ball pops up, replacing the default too cursor/crosshairs, when using the highlighter, pencil or eraser tool longer than a few seconds. And, the beach ball doesn't go away until you've released your your mouse. This affects the precision and accuracy of the tool being used. In our situations, we use Acrobat for marking up copy and the beachball will cover entire letters not allowing the users to accurately draw/markup.
User's Computer
27" iMac w/ 5k Display
3.3GHz, i5, 16GB Ram
OS 10.10.4
Acrobat XI (11.0.10)
Computer 1
27" iMac
3.5GHz, i7, 16GB Ram
OS 10.9.5
Acrobat Pro DC (2015.008.20082)
Computer 2
15" MacBook Pro w/ Retina
2.6GHz, i7, 16GB Ram
OS 10.10.4
Acrobat XI (11.0.10)