With WHM on VPS it has whatever version of PHP you want including no PHP.
Go to Easy Apache , you can install php7.4 which has improved performance over 7.3. Then go MultiPHP manager and you set default system version and change the version for the domain. Go back to easy Apache and you can uninstall older versions of PHP.
If you have free RAM you may also want to consider installing and configuring OPCache. Easy Apache only installs it, it does not activate or configure it. It caches your php files in RAM and can dramatically improve performance. I did not mention this before because you need to allocate RAM to it.
Once you get that squared away look into using Sphinx as search backend, it's lighting fast but once again you need to allocate RAM to it.... welcome to the world of you can do anything you want.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison