With that setup you're pumping 8.3 million pixels
That is 4K terratory which is 8.3 million pixels.
Unfortunately you'll be dealing with more triangles that can't be discarded so the overall render will be slower as the early discard phase will reject less.
Sorry, I would go no less than 2080ti. 2080 and 2080ti are the only real 4K solutions. And the 2080 is debatable because it's only 8 Gigs RAM. I've seen games push 10GB @ 4K
And I would bump that 2600 up to a 3600 or 3700.