Performance problems are tricky to diagnose, but you can help by providing a sample. This is also useful if you get a beachball.
There are two ways to get a sample. You can use Activity Monitor or run /usr/bin/sample.
Activity Monitor
- Open activity monitor (in /Applications/Utilities)
- Select iTerm2 in the list of processes
- Click the "i" button
- Click the "Sample" button
- Save the result and attach it to the issue.
/usr/bin/sample
Activity Monitor can be tricky if your performance problem is short lived. You can capture a sample at the command line (either in iTerm2 or Terminal.app) by running /usr/bin/sample $PID > sample.txt where $PID is the process ID of the main iTerm2 process. You can use Activity Monitor to find the process ID by searching for iTerm2 and use the value from the PID column.