I've been having this problem for a while, and its been extremely annoying, but I think I've found a solution, at least for my machine.
I noticed that when I had terminal closed, the sublime cursor moved much more quickly. After further investigation, I determined that I by turning off the "blur" affect on the background of terminal, sublime would stay just as fast even with terminal running. This issue also only seems to appear when my mac is running on integrated graphics, so I'm guessing that it is a hardware limitation of the integrated graphics card that slows down the compositing process. Terminal also runs much faster with blur disabled, so I think I'm going to keep it off permanently now.
A few other interesting tidbits:
- Sublime's performance only degrades when I set the terminal blur to above 62%
- If the terminal window is completely covered by sublime, no performance hit occurs
- Its less noticeable, a maximized, blurred terminal will also affect the scrolling performance of Safari and other applications