@Staffand @benfitita , thank you for helping me. After a lot of troubleshooting, I believe I have discovered the cause of the issue. Even though I use the same Docker image for all of my VPN connections, I use each VPN container differently. In particular, I mostly use one VPN container connected to an AirVPN server when I need a proxy. The WireGuard slowdowns seem to have been caused by my usage of Privoxy in conjunction with a low memory limit of 70 megabytes.
I resolved the issue by removing the memory cap on my VPN containers.