A slow VPN is usually caused by one of the following things:
To troubleshoot bandwidth issues:
Check that you have enough bandwidth with a network speed test.
If you are using a PPPoE internet connection, check that the MTU size is correct for the VPN.
Try setting the interface settings to use the exact speed of the network connection, instead of automatically detecting the network speed.
To troubleshoot hardware issues:
Check that there is enough RAM and CPU for the router terminating the VPN connection. You may need to disable some additional services (like gateway antivirus) or upgrade your hardware to fix slow VPNs caused by hardware issues.
If using a VPN client, check the performance of the PC.
To troubleshoot software issues:
Try updating the firmware of your router.
Try updating or reinstalling your VPN client if you are using one. (Check with the manufacturer what versions work with your version of your router)