If your fan is spinning up when scrolling in Slack it's likely an indication that Electron (Chrome) is refusing to use the GPU for rendering acceleration. This is likely either due to a driver issue or the driver/gpu being on chrome's blacklist. I had this problem once on a hackintosh and as I recall starting Slack from a terminal with the `--ignore-gpu-blacklist` option fixed it.
Almost all the pixels on your screen are rendered through a GPU accelerated pathway, if they weren't even the desktop with no apps open would be rather sluggish.
