OpenVPN transfer performance

I’m running a VPN server on my Synology NAS through OpenVPN. My issue is the transfer speeds from remote users trying to download from the NAS seem pretty slow.

I’m currently getting 400 Mbps upload and download through my ISP. But the transfers are averaging around 8-10 MB/s.

I’ve read a few articles that have suggested changing the MTU parameters. Has anyone done this for their OpenVPN server hosted on their Synology NAS? I don’t see anywhere to change MTU settings on the server menu options or on the client config file.

Also open to other suggestions on how to improve transfer speeds for remote users.