You will not be able to find a 2560x1600 resolution monitor faster than 5ms or 6ms because they all use either S-IPS/H-IPS/P-ISP LCD panels. The short answer is that these types of panels cannot go below 5ms because of the number of colors they produce; 16.7 million.
Less expenses LCD panels (TN and e-IPS) do not "naturally" create that many colors which is why they have lower response times; especially TN panel monitors.
========= Long Explanation ==========
S-IPS/H-IPS/P-ISP LCD panels are known as 8-bit panels because each primary color (Red, Green, Blue) have 8-bits that are used to display colors. They are less often referred to a 24-bit monitors. All inexpensive monitors basically use 6-bit color panels (TN and e-IPS) panels; each primary color only uses 6-bits instead of 8-bits. They are far less expensive to manufacture.
What's the difference between 6-bits and 8-bits? It is beyond huge. Basically we are dealing with binary math here. The number of shades for each color is represent by either 2^6 or 2^8. That is 2 to the power of 6, or 2 to the power of 8. 2^6 means you multiply 2 by the number 2 six times. 2^8 means you multiply 2 by the number 2 eight times:
2^6 = 2*2*2*2*2*2 = 64 shades of color
2^8 = 2*2*2*2*2*2*2*2 = 256 shades of color
Each primary color in a 6-bit panel monitor has 64 shades. That means a 6-bit monitor actually produces 64^3 = 64*64*64 = 262,144 colors on the screen.
Each primary color in an 8-bit panel monitor has 256 shades. That means a 6-bit monitor actually produces 256^3 = 256*256*256 = 16,777,216 colors on the screen. An 8-bit panel is capable of displaying 64x more actual colors than a 6-bit panel.
All 6-bit panels (TN and e-IPS) are advertised as capable of displaying up to 16.7 million colors because they use a technique called temporal dithering. Say for example a TN panel cannot produce purple by itself; it is one of the 16.5 million colors that it cannot "naturally" produce on its own. This is solved by temporal dithering which means the pixels pulses extremely fast between two colors to create a third color. In this case Red and Blue creates purple, so the pixels pulses between those two colors so quickly that it appears to be solid purple on the screen.