When using the brush tool at full screen (Fill mode not 1:1) for extended period (10-15 minutes) I begin to experience a lag in screen replenish and often get the Mac beachball. The entire operation of the Dev/Brush tool slows and gets unmanageable. I usually close LR and restart but the problem returns. I turned off the GPU support as that exacerbated the problem.
Image is 21MB Canon Raw file.
1:1 and Smart Preview was built on Import.
Using brush tool at low flow (40%) and density (100%) so there are a lot of 'strokes' applied in B/D of areas of image
File cache currently set at 20GB
Running late 2012 Mac Mini (Quad Processors/I7-2.3ghz/16GBRAM) and with Dell U2415
Original images located on W/D 4TB external drive; LR Catalog on Mac 2TBHD-256GB SSD/Fusion drive