You can't stop it but with real-time scanning off all the time you might as well uninstall the software as it's basically useless apart from whatever protection the firewall side of it affords you.
Also the software polls for updates every 4 hours and actually updates at least once every 24 hours, hence the Systrem Requirement of Internet access.
The consumer/home software is not designed the same way, say, as Enterprise software which is more configurable for use in special circumstances.
That all said, it shouldn't be slowing you down to any great extent unless you are downloading risky software continually or something like that.
Try turning off Netguard under Web and Email Protection > Firewall - see if that speeds things up.
Have you thought of opening a case with Technical Support - see Useful Links at the top of the page?