What does your profiler tell you? If you don't have one already, get PIX here. It can look at your CPU and GPU performance and tell you where your program is spending its time.

I am not well versed in modern cpp or dx12, but it seems you are hammering the api too much. Your code reminds me of my old opengl code, which basically relied on opengl to perform basically everything. This was a standard - and wrong - practice back then, but nowadays, people are usually try to separate every api related functionality and implementation into a separate file, and they try to keep it extremely simple and short, so if there is something wrong (or they want to add other api), they can instantly relatively easily can solve the problems. Meanwhile your code looks like very reliant on the api and on the compiler itself, i am having a hard time figuring out what is supposed to do what.

So what i am recommending you is - instead of trying to hunt down this particular performance bug - to reorganize everything while you can, and then maybe the bug will automatically reveal itself.