I have confirmed a bug in the latest version of Pulse Secure VPN with macOS Mojave (regardless of what version (10.14.0 - 10.14.3 have the same results). Regardless of whether you are currently connected to VPN or not, the app appears to be the culprit of a major network slowdown.
Behavior: If you connect to VPN and then disconnect but the app is still running in the background (which is the default and expected) network speeds on WiFi or Ethernet are significantly slower than normal if you are on any version of macOS Mojave. By slower, I mean, you normally get 850-900mbps on Ethernet and you get 50-100mbps. On WiFi, you normally get 150-200mbps and you are getting 10-20mbps.
Temporary workaround: Quit Pulse Secure VPN entirely by clicking on the icon in the toolbar and selecting Exit. You will be prompted if you are sure. Click OK.
I have confirmed at least 5 cases within my organization (which is still testing Mojave for production level support) and confirmed this discovery when reaching out on the MacAdmins Slack workspace and found fellow Mac users with the issue and the one commonality was Pulse Secure v9.0.2 or v9.0.3 installed.