I noticed when i run streams that it's bitrate is close to my max Internet speed it lags a lot when using VPN but run smoothly without VPN. Bandwidth monitor programs shows that the speed through the Ethernet when running the streams using vpn is at max so the play should run smoothly! So, does vpn encryption(or vpn in general) consume part of the traffic?
Encryption doesn't blow up the size of a data stream. There's some setup overhead but it is negligible. The lag you're seeing is most likely due to the CPU cycles required to do the encryption and decryption at the ends of the VPN circuit. The CPUs involved might be unable to encrypt/decrypt at your full link bitrate.