I would split optimization into two parts: TTFB (time to first byte) optimization and the frontend optimization.

To optimize TTFB:

• Connect your Magento store to a PHP profiler. There are several options, you can google for them.

• The profiler will give you a diagram like this one:

• Inspect the diagram and see if you have find a function call that takes too much time.

• Optimize that function call. In 90% case I dealt with the slowness came from a 3rd-party extension.

To optimize the frontend:

• Minify and compress JS and CSS. You can turn it on at Stores > Configuration > Advanced > Developer > CSS and JS settings

• Serve images in WebP or AVIF formats to cut page weight

• Use GZIP compression

• Inline critical CSS and JS (critical CSS/JS is what needs to be render above-the-fold content) and lazy load all the rest

• Use as few 3rd-party JS libraries/scripts as possible

• Remove redundant CSS and JS

Good luck!