In Google Chrome (current version: 96.0), there's a developer toolbox; in that toolbox, there's a tab called 'Lighthouse'.  Lighthouse is a tool that can assess the performance of web pages and makes recommendations on how to speed them up.

It had a lot to say for Jira!

Most notably, there was notice at the top of the report:

There were issues affecting this run of Lighthouse:
* The page loaded too slowly to finish within the time limit. Results may be incomplete.

It gave a performance score of 38/100, and the page took 8.2 seconds to become interactive.

There are too many recommendations to list here, so you'll probably want to run it yourself.  It's very informative and the recommendations are clear and concrete.

Hope that helps you; I know it's helped me a lot on my projects!