OK I think Jules and I may have a solution to this problem. It is related to the laggy repainting bug since OS X El Capitan. Apple seems to have changed something in their repainting code since El Capitan which makes redraws extremely laggy if you post a lot of setNeedsDisplays.
The same bug is affecting Logic: you can clearly see how the GUI struggles to keep up when moving a JUCE slider. In addition, and more worryingly, Logic seems to be time-stamping and recording parameter events on the UI thread - and therefore the laggy UI seems to affect the timestamps of the recorded parameter changes.
Please let me know if the recent change on develop fixes this bug.