OVPN's desktop client has a feature named Use strongest encryption, which is enabled by default.
The setting controls which encryption algorithm is used for the VPN tunnel. If enabled, the AES-256-GCM encryption is used.
However, older computers or computers with slower processors may experience low internet speed if AES-256-GCM is used. Thus, it is possible to reduce the encryption to AES-128-GCM by disabling the setting.
AES-128-GCM is still safe to use and there are no known vulnerabilities.
The setting is enabled / disabled under the Settings tab of the client.