I never noticed it previously, because SM always launches to fit the screen, but I do see the lag when resizing. I have a feeling this has something to do with Git History sidebar, if I vertically resize window to hide 90% of the history bar then horizontal resizing is very smooth. But once I fully expand window vertically, and all history is rendered resizing is very slow.
I'm at the build 1084 at the moment, theme is Light (I'm currently evaluating), macOS theme is Dark. System info is below.
System Version: macOS 10.14.1 (18B75)
Model Name: MacBook Pro
Model Identifier: MacBookPro13,3
Processor Name: Intel Core i7
Processor Speed: 2.6 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 6 MB
Memory: 16 GB
Boot ROM Version: 250.0.0.0.0
Please let me know if there is something else useful that i can provide.