NS Throttling Internet Connection

@EddieA, could you please carry out some more tests to find the culprit?
I'd like you to enable IPS on the server-manager choosing whatever policy you want, then stop snort via cmdline with:

service snortd stop

Then, please, repeat the speed test.
If speed goes back to normal, we could be pretty sure that snort is throttling network performance.
From there, I'd try to find a snort option to improve performance.
That means restarting snort with different options and repeating the speedtest every time.
The option I hope will make a difference is:

config detection: search-method lowmem

in /etc/snort/snort.conf.
You could try to change lowmem with ac, ac-q, ac-bnfa, ac-bnfa-q, ac-split (see http://manual-snort-org.s3-website-us-east-1.amazonaws.com/node16.html for all options).
To sum it up:
1. service snortd stop
2. change snort.conf
3. service snortd start
4. speedtest
5. repeat from the beginning

Thank you very much.