your hardware is enough to cater plenty, only issue is NUMA configuration, because you have 2 CPU's therefore you have to configure MySQL to access memory properly, this is why the LOG-IN that hits the database is showing slow response.

you can check this blog post to setup NUMA on MySQL.

Alternatively you can rub the Magic Lamp for Ken Task to show up, he is MySQL Guru on Linux to better guide you

What was wrong with your other post https://moodle.org/mod/forum/discuss.php?d=399461 ? Please don't repeat the same question!

@Usman ... cute ... but have never claimed to be a 'genie'!

But ... can dig and ask questions ....

@wadel

From link provided for NUMA ...
"NUMA imbalances in MySQL can cause extreme swapping."

What does top output show?

From what I read about NUMA it's about NUMA nodes and don't think
from his server config description, there are any nodes???

numactl.x86_64 : Library for tuning for Non Uniform Memory Access machines

On a standalone CentOS 7 (MySQL 5.7) output of numactl checking hardware shows:
[root@server ~]# numactl --hardware
available: 1 nodes (0)
node 0 cpus: 0 1 2 3 4 5 6 7
node 0 size: 8187 MB
node 0 free: 313 MB
node distances:
node   0
  0:  10

No multiple nodes.

cPanel - is commercial and
"the server installed outside the campus. (in Netherlands)"
suggest hosting with a provider.  What provider and why
would country be mentioned?

Guest login should be testing a manual account in the Moodle.
In mdl_user table, ID # 1

So query DB ... for guest account info from the mysql client ON the
server:
mysql> select * from mdl_user where id='1';

LDAP authentication is more often to a Windows service so
it's config and networking between Moodle server and the LDAP service on remote server is involved.  Settings for LDAP could be ldap:// or ldaps:// the latter using encryption of data being exchanged.

From the moodle server CLI, do some network test.
Can you ping ldap server?
Install nmap on moodle server and scan ldap ports used:
nmap -P0 -p [portnumbers separated by commas] [IPaddressofLDAPServer]

Issues could also involved firewall on both ends.
Moodle server running mod_secuity or securi that has rules being checked when data is exchanged?

Even if benchmark used a mysql test, think I'd still install MySQLTuner and run it with DB server superuser credentials.   Suggestions it has for tweaks to config are usually beneficial.

https://github.com/major/MySQLTuner-perl

Soooo .... ?

'SoS', Ken