IMHO you really should not be experiencing any lag. I have no input lag at all using the following:
Intel i7 3770k no OC Nvidia GTX1060 6Gb (RS graphics maxed) 16 Gb RAM Onboard sound. Win 10 Pro 64 bit
My C: is a 450Gb SSD and my storage drives are 1 TB Seagate Hybrid drives. You system should smoke mine.
I dont use the ingame instrument sounds however so I dont know if that has anything to do with it. I pretty much gave up on getting an interface to work smoothly with RS and use my pedal board. I plug my guitar into a standard cable that plugs into my tuner, then goes to a compressor/sustainer pedal, then to my MXR M80Di. The MXR has 2 outputs on it. One I plug the RT Cable into and the other goes to my Eden EC10 Amp or to the head unit depending on how badly I want to annoy the wife / neighbors.
The game sound I pipe out of the PC into a Techniques 7 band EQ and into a set of Mackie CR3 Creative reference monitors and a Polk Audio powered 10" Sub.