Well, the screensaver is not the issue here. And probably not the build either, as you say, as I tried a few random nightly coreelec builds going back to around 1/1, all with the same issue.
Define userspace implementation? I use the wireguard app on my clients (Android phones & tablets + a Win 11 PC, all with the same issue) and serverside it is set up as described here: WireGuard VPN tunnel configuration using wg-quick script
Testing methodology is connecting, and then running a ookla speedtest or copying a file from the storage connected to the box.