Unstable slow connections, interruptions. Turn off browser and turn on and is is normal for awhile. Problem started when I started using F-Secure
We would like to have more details about OSX version running on your iMacs as well as the affected browsers.
Meanwhile you may want to uninstall and reinstall F-Secure SAFE then check if the connectivity improves.
We suggest you contact our support team for some troubleshootings on your devices in other to isolate the possible rootcause.