[Pre-flight] [Frontend] Performance audit and optimization for Rapid Diffs on MR Creation Form

Context

As we approach the final steps of Rapid Diffs:New MR Form deployment we should do a performance check against the acceptance criteria of the project and either simply identify bottlenecks or also fix them.

Timeboxing at w2 for backend and frontend.

This issue is focused on performance.

As a Stretch , we can check all of the acceptance criteria including Accessibility compliance.

Verify 2 baseline scenarios:

1. Loading the page with Rapid Diffs
2. Interacting with Rapid Diffs (collapsing files, resizing file browser, etc.)