Decline in speed, how to test the problem?

Question

Computer networks

There are Kerio + SNR switches, a provider giving Internet speed of 30Mbps, the situation is as follows, 45 employees using the Internet, I decided to measure the speed the other day, and there the speed (in the location where the provider is located) ping 2ms, input. speed varies in the range from 17 to 23 Mbps, ref. varies from 18 to 24Mbps.
How can you understand why such drawdowns can be?

Answer 1

30Mbit, the situation is as follows, 45 employees using the Internet

17Mbit is still not bad, more than 50% of the free bandwidth, these employees somehow lazily use the Internet.

Answer 2

Disable all access to the Internet.
Connect only Kerio there. measure speed