Allowing more than 20 concurrent connections?

I have a server running WHM/Cpanel with 16GB, 8 CPU processors, and I've been having problem with multiple accesses concurrent, it seems it is not allowing, this is the Apache Configuration I set on Apache through the WHM (and right after, restarted apache).

- Removed -

The configuration is set correctly, but it still doesn't allow more than 20 multiple connections, I'm running a simple PHP script for testing.

test.php

<?php sleep(100); ?>
And then I run in the browser 20 tabs of this same page, so it keeps connected running multiple instances.

And then I run the page simple.php on a browser in another computer using another internet connection

<?php echo "Hello"; ?>
And it hangs on a lot to load the page (it is probably waiting in a queue, instead of just running), and when I close 10 of the tabs of the test.php page, the simple.php runs instantly as it should, but if I reopen 10 tabs of the test.php, same thing happens, the simple.php takes a lot of time to load.

My server gets a lot of access but it gets so slow to load the pages when I'm having multiple accesses. (even with plenty memory free and CPU Usage percentage very low).

What should I do to allow more connections without apache queuing the HTTP PHP Requests?
Do I have to set the serverlimit,maxrequestworkers for each specific domain account in the WHM, if yes, where do I do that, I only have one domain account in my WHM.
Thank you very much!