Very slow TTFB or White pages (error 500?)

PHP Warning: Module 'mcrypt' already loaded in Unknown on line 0
It means you have mcrypt.so loading in more than one file. Check virtualserver and global (main) php.ini files for the line containing mcrypt.so and comment out that line (or lines if more than one). Then restart Apache.

[fcgid:warn] [pid 17904] mod_fcgid: process 18461 graceful kill fail, sending SIGKILL
This error shows when your script was running more than allowed and then the server kill the process. This is more complicated how to solve. It could be low settings with fcgid, server running out of memory or cpu (oversold server), bad coding, wrong MySQL configuration or all combined. I would start with fcgid configuration especially numbers set with FcgidBusyTimeout, FcgidProcessLifeTime and FcgidIOTimeout.
Just remember that increasing this values will not help if the problem is somewhere else (like bad coding) and could be short term solution but in the long run can bring so many problems to even put your server down.
Not sure if this is same with every distro but Virtualmin during the installation could set some fcgid values inside httpd.conf file. If this is your case my advice is to remove them from httpd.conf and set them within fcgid module. Why Virtualmin is doing this i dont know, but i notice this was (still is?) the case with Centos 7. Not saying that is wrong but it complicates things when is no need to be complicated, e.g. settings instead to be set inside the module are displaced all over the place and makes it easier to make mistakes during editing.

For TTFB is even more complicated because factors what can affect are so many, from hardware used (HDD vs SSD), Apache and MySQL caching (enabled / disabled, and what settings), DNS (local or 3rd party), CDN quality (if used), settings with Apache and MySQL, what storage engine for MySQL (InnoDB, Myisam...) and so on. Could be even slow because of other two problems you mentioned. Its really hard to pinpoint the problem without deeper analysis of your server and reports from several testing tools.
For example if you check my post https://www.virtualmin.com/node/48280 for GTmetrix pictures and compare with new results it is clear how changing several settings can and will affect your website:


My TTFB from 0.7-0.9 seconds dropped to 0.4-0.5 (sometimes 0.6) seconds. The differences are not big but its like making a mosaic, one damaged piece by himself isnt a problem but when you combine more of them the final picture will look bad.

Thats correct. Apache default settings are usually good for a blog about cat or dog with few visits per day but actually they are pretty bad and useless, even the one changed during Virtualmin installation (e.g. when you need to select memory amount).