This is a User to User forum, where fellow Plesk users will respond if they have a useful comment to make on a post. So don't expect instant answers, or any answer at all. But usually people do respond as long as the post is sensible and polite, and they have something useful to add.
Firstly, the File Manager can only access files that are owned by the hosting account user (typically the username you selected when creating the hosting for domain), and usually the files in the /var/www/vhost/domain.tld/conf directory are owned by root. In any case, other than vhost.conf, you should not directly edit stuff in there.
The main apache configuration is in /etc/httpd/conf/httpd.conf (for RedHat/Centos type OSes and elsewhere for others).
To edit this and other configuration files to make global changes you would normally use an SSH client like PuTTY to connect to your server directly. You may then have to change to the root user. How to do this sort of thing, which user to use and so in is something that the company that provided your server should be able to help you with if you didn't do the install of the OS yourself.
Note that slowness in one thing may be related to a problem in something else. In other words it isn't necessarily apache that's the problem. However, the out of the box apache configuration is not necessarily ideal for all situations so it is certainly somewhere to start looking.
But before you do anything else, check your logs for errors. Not just the apache log, but the main system logs too.
Does your site rely on a database? If so then database configuration tuning is something to look at (and database logs).
Also make sure your dns server is configured correctly and working as expected.
There's an awful lot to look into if you are unlucky and don't find the cause of the slowdown easily/quickly, so have patience. Take a look at utilities like apachetop and apachebench (ab) to see what apache might be doing or not doing. You can install these from the command line, though you may need to obtain them from a third party repo or compile them from scratch.
Also read and re-read the Plesk manuals - particularly the admin one - in detail. There's lots of useful stuff in there but it is a big document and difficult to digest in one go.
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