Norton VPN cutting my internet speed by more than half

I pay for internet speed of 500mbps. If Norton VPN is on I get speeds, for example, of 144mbps download (Thinkbroadband) and 157.74mbps download(Speedtest.net). If I turn the VPN off I get downloads of 516.57mbps (Speedtest.net) and 454mbps (Thinkbroadband)

I should like to be able to use the VPN but also want to be able to use the internet speed that I am paying for.

I have read previous posts about this but could not find a solution. Is there one, or should I just turn off the VPN?