Internet speed getting slower over time on MBP
In my case this was caused by the Juniper/Pulse Secure VPN client's kernel extension, which was active even when not connected to the VPN. Unloading the kernel extension restored speed without a reboot.

Short term fix is to unload the extension, command copied from linked kb article:

sudo kextunload /Library/Extensions/PulseSecureFirewall.kext


Long term solution is to upgrade pulse secure client. I was experiencing this problem with pulse secure 9.0.3(1599). I upgraded to 9.0.3(1667) and the kext is no longer loaded by default. I can connect to the vpn without the kext loaded, and speed is no longer tailing off. Solved!

With pulsesecurefirewall.kext loaded:

Without pulsesecurefirewall.kext loaded:


If your VPN security policy requires "Lock Down Mode" or "Traffic Enforcement", you may need to downgrade instead of upgrade the client. 

For future reference, you can view loaded non-apple kexts with:

kextstat |grep -vi com.apple


Edit: What is a kernel extension?
A kernel extension is a program that runs within the macOS kernel. Running within the kernel can give performance improvements for certain applications, and also access to APIs that aren't available to normal (user space) programs. 

In this case, the pulse client included a kernel extension which provides firewall functionality by inspecting packets as they pass through the kernel. The provided extension is buggy and performance drops over time (maybe it keeps a list of something, and as that list grows, the length of time taken to scan the list increases, so the time to process a packet increases, leading to a drop in throughput??)

Unloading the kext removes the buggy code from the kernel, so that it doesn't affect performance.