Generating... Takes incredibly long

Generating the changes took like 10 second, but once I click apply, it took like 5 minutes to fill the generated changes into my file, way slower than if I were to fill in the changes myself. I am wondering why is the gray bar moving so slow.

Is the file particularly long? I noticed that the apply changes feature got significantly slower once my script exceeded about 600 lines or so. I’ve been good about keeping it under 1200 lines but I don’t think it’s usable if it got any longer than that (unless you stick to the ‘apply changes to X’ features which have gotten a bit better).