So for the past 6 months or so this issue has been happening. Latest Sierra on late 2012 iMac. When using Safari in a rather normal manner or for work i will have many tabs open. The issue can happen with anywhere from 6 or more tabs open. If Safari sits, meaning i dont refresh a page or anything and then i go to refresh a page or open a new tab Safari makes the system freeze/lag/skip for a split second. My mouse will skip. If a video is playing on VLC, Quicktime, Youtube they will also freeze for a split second. I have cleared Safari. I have tried removing pinned sites. Enabling/disabling app nap. Safari is the only browser that causes this issue.