VPN on one Mac much slower than other Macs. How to troubleshoot?

I have two MacBook Pros that are giving me about 24 Mbps down when connected to my employer’s VPN (it’s a natively configured VPN). My third MacBook Pro is clocking in at only 8 Mbps down with the same corporate VPN. What are some good first steps to troubleshoot the problem, assuming it’s a client-side …