[Performance] Insomnia is laggy when working with large request body strings

- Insomnia Version: 5.14.8
- Operating System: Reproducible on Windows 7 and Windows 10

When working with large strings in request bodies, the request body editor in Insomnia becomes very laggy. These are some of the operations that all take multiple seconds to execute:

- Pasting a string
- Deleting any character in the string
- Selecting all request body with CTRL + A

The lag is clearly visible with a single ~100 KB strings. Using a single ~1 MB string, it is pretty unbearable, with every operation mentioned above taking multiple seconds to execute.

Start by reproducing the lag in Insomnia's request body editor with approximately 100 KB and 1 MB strings, testing paste, character deletion, and CTRL+A. Trace the editor's handling of these operations and measure their responsiveness; done means the same operations no longer take multiple seconds.

Written by the indexing model from the issue text.