If you are not getting the Internet speed that your ISP has promised, of course they will first ask you to test on their side of your Kerio Control Firewall. If that test shows that they are delivering the promised speed, the problem is at the firewall, obviously.
However, is it really a "problem"? After all, the purpose of the firewall is to scan packets and make decisions. Those decisions take time and that slows down your speed. You could turn off various functuons to improve speed, but that just turns off the protection. The protection afforded by that scanning is why you have a firewall, so turning off features really doesn't make sense.
It could be due to a NIC speed mismatch. The default is to autonegotiate, but that can sometimes be incorrect, so you can set it manually.
If that's not the issue, it's probably your CPU that is underpowered. To see this for yourself, you might momentarily disable IPS as shown here:
That will make an immediate and large improvement. Of course you still have virus scanning and traffic rules, but most of the drag caused by an underpowered CPU will go away with IPS shut off. Don't leave it shut off - upgrade your CPU or if running the smaller Kerio Control Hardware Box, upgrade that (licenses can be transferred in that situation).