UDP is different from TCP. It doesn't have Window Scale but it doesn't need it OS-wide, but low buffer sizes may slow down it, too. If you think OpenVPN is still slow after changing buffer values to 0, you should either increase OS-wide buffer sizes (net.core.rmem_default and net.core.wmem_default) or increase buffer in server config:
Jun 10, 2020 Fix problem with Norton Secure VPN screen if it stays at Jun 26, 2020 Why is my Wi-Fi slow? How to fix a slow internet
Feb 03, 2020
Fix problem with Norton Secure VPN screen if it stays at
Is Your VPN Connection Very Slow? Here's How To Fix it
Using a VPN doesn’t have to slow your internet speed down noticeably. Usually, any impact on speed is minimal if you’re using a good VPN service. Free or untrustworthy VPNs may be slow or inefficient due to server capacity issues, location issues, or poor server infrastructure. Why does VPN slow down my internet speed? The prime reason why you’re experiencing slow speeds is because you’re connected to a VPN server that’s too far away from your actual location. For instance, if you’re located in the Middle East and connect to a U.S server, you’re going to notice a significant speed drop. How to solve slow speed issues? – IPVanish