pretty vague explanation.
1. Fresh Windows install?
2. Was working fine before?
(if so, what changed)
3. What games?
4. EXACTLY what do you mean by lag?
Some tips (if not done):
1. Windows needs to be installed from scratch (if new motherboard socket).
2. test system memory www.memtest86.com (full pass or until errors... use "XMP" setting for memory in BIOS first)
3. NVidia drivers from NVidia site (if new GPU but not motherboard then use DDU to remove old drivers. use "recommended" setting
http://www.guru3d.com/files-details/display-driver-uninstaller-download.html
4. Power Settings-> make sure PCIe-> link-state isn't on HIGH (can throttle PCIe bus and thus the graphics card)
5. TEST with benchmarks and other repeatable scenarios.