Slow connection from home to work

Background:
- Work Internet: AT&T Fiber (Business) 250mbps synchronous
- Firewall: Fortigate 61E
- My Home internet: xFinity cable internet 800Mbps down / 25 Mbps up
- Speed test results: 532 / 42 Mbps

Some of my users are complaining that file transfer is slow when they transfer files from the work machine to the home machine via the VPN. I decided to test it out myself. I did a speed test with both Google and Speedtest.net and got a download/upload speed of roughly 500/42. So home internet is fast. I did an iperf test with the file server acting as the iperf server. The file server is behind the firewall. When I VPN into the network and do an iperf test with my home machine as the client, I get speed of 38mbps. So I am thinking, it could be the VPN. So I did the same test, except this time I configured the server so my home machine can connect to the file server without VPN but it still goes through the firewall. I get a speed of 41mbps. OK, I am wondering if it's the firewall that could be causing the problem, so I set up a Windows 10 machine acting as an iperf server outside of m y firewall (with a public IP address). Testing that against my home machine, I still only get speed of 41mbps. I have another coworker run the same test. The result of all the tests are below (initials are name of the users):

MN: xFinity 800/25, Speedtest Results 532/42
home workstation to filesever (with VPN) - 38mbps
home workstation to fileserver (no VPN but through firewall) - 41mbps
Home workstation to work computer (no firewall. Direct connect to the internet) 41mbps

MW test: xfinity. Speedtest results 250/15.
home workstation to filesever (with VPN) - 17 mbps
home workstation to fileserver (no VPN but through firewall) - 18mbps
Home workstation to work computer (no firewall. Direct connect to interthe net) 18 mbps

EC test: ATT fiber. 800/800. Speedtest results 630/510
home workstation to filesever (with VPN) - 52 mbps
home workstation to fileserver (no VPN but through firewall) - 227 mbps
Home workstation to work computer (no firewall. Direct connect to interthe net) 229 mbps

First: Why is the connection from the home machine to the (MN), why am I not getting closer to 250mpbs?

For the EC test, the transferred speed slowed down considerably when connected via VPN. I am guess that's a VPN issue possible. I will work with the vendor on that. But I am still curious why I am not getting closer to 250mbps.