Improve browser Largest Contentful Paint (LCP) performance of Project File Rendered (Markdown) page into S3 tier

The Project File Rendered page's browser Largest Contentful Paint (LCP) performance is unperformant:

* Environment:                10k
* Environment Version:        16.2.0-pre `84ff1bd549e`
* Date:                       2023-07-21
NAME                                  | FCP (ms) | LCP (ms)       | TBT (ms)       | SI (ms) | LVC (ms) | TFR SIZE (kb) | SCORE | RESULT
--------------------------------------|----------|----------------|----------------|---------|----------|---------------|-------|-------
web_project_file_rendered             | 2139     | ✘ 7208 (<6000) | ✓ 2045 (<3300) | 3740    | 7217     | 1693.7        | 70    | FAILED

The Project File Rendered page being tested can be seen here on Staging. This page is showing rendered markdown file, below is the screenshot of page's historical test LCP results

Note that performance results spike intermittently, tentatively due to the cache - see https://10k.testbed.gitlab.net/-/grafana/d/jH6yoWVMz/gitlab-performance-sitespeed-results?orgId=1&from=now-90d&to=now&var-test_name=web_project_file_rendered and discussion #331447 (comment 1286430610)

Test Details

Testing was done on our 10k Reference Architecture environment with our lab condition SiteSpeed pipeline (named GitLab Browser Performance Tool internally). The project being tested is a copy of gitlabhq (tarball can be found here). GitLab Browser Performance Tool tests information is listed at Current test details page.

The latest GitLab Browser Performance pipeline results can always be found here. Through this page the full SiteSpeed report can be found via the SiteSpeed Report link on that page.

As per our performance targets this page's LCP metric is above the target of 4000 ms which is severity2 . Task is to improve the page's browser performance into next tier.