Here is a guide to fix a slow Magento 2 site.
First of all, you need to optimize time to first byte (TTFB) and for that you'd need to use a PHP code profiler. It will tell you exactly what code is slowing the site down.
Here is an example of a code profiler graph with code files and times:
Magento 2 comes with a built-in profiler but its functionality is somewhat limited. You can try an online profiler (tideways, new relic or similar, just google for it), run traces for slow pages and see what's going on. It's pretty self explanatory and it really helps.
Secondly, try a 3rd-party module audit. I performed several optimization services and in almost all cases, it's some custom plugin that slowed the site down. Try turning all modules off and see if it makes a slow faster. If it does - turn them on one by one and see which one(s) breaks performance.
Thirdly, switch to a luma theme. Does it make a difference. If it does - then you know, your theme is a problem and it needs to be investigated.
Once you have TTFB down to as small number as possible you could try frontend optimizations.
Besides, CSS/JS minification and merging, try converting all images to WebP and AVIF formats - it really makes a difference on mobile platforms. Move JS to the bottom and remove redundant CSS with the help of Google Chrome Coverage tab.
Good luck optimizing!