Troubleshooting slow server response times

[strarsis]

First rule out DNS issues, how fast does dig look up the IP address of the site? Use ping and check for network issues (short ping times, variations).
Then use a tool like wget or curl and check the response time reported by these, are those really that large? Do they vary a lot?

(Besides profiling) I achieved significant performance improvements by using an object cache (Redis based in that case):

Use Redis cache with Bedrock and Trellis

Maybe this is not caused by the object cache but by nginx microcaching? More rarely changing pages are held in cache for some requests, although they may need rerendering. Pages that are individualized or often-changing (as WooCommerce cart, check-out, account pages) should be excluded from microcaching: Do you have the same issue when you disable the object cache?

PHP 8 also improved performance compared to PHP 7.x.