[MovedGoalPosts]

I'd suggest that needing a separate NIC is a last resort.

We don't have a lot to go on here and need to determine what "lag" actually means. Is it really that information is taking too long to get between PC and server, and back again? Is it some other issue on the computer that means the response time of what is seen on the screen is slowed?

Rule out the computer. No antivirus getting in the way? Appropriate drivers installed for all hardware? Graphics settings at reasonable settings so the card can actually cope. Nothing else running that is eating resources?

Now look at the network. No other kit creating traffic locally? Are you actually getting the claimed speeds? Try tools such as that from thinkbroadband that can monitor latency.