Hi Beesho99,
Yes, your mouse is most likely not set correctly and you are experiencing hitting the game's maximum turn speed as described here.

Let's get some information so we can better help you.
What mouse are you using?
What DPI is the mouse's DPI setting?
What is the KeyMander's DPI setting? (it should match your mouse)
What are your mouse sensitivity settings?
What are your in-game sensitivity settings for horizontal and vertical?

PICTEK mouse , the most dpi is 7200 , and keymander dpi settings is 500 , my mouse sensitivity settings is 60, and sensivity in the game vertical is 70 and horizontal is 68 , please can u give me the best settings for no aiming lag ?? Thanks

I am not familiar with that mouse, but you need to set the mouse to 7200 and also set the KeyMander to 7200. Are you using our sample profile for R6S?

I need to set the mouse 7200 and aiming lag will be gone or what ? Can u send me ur sample profile ?

As I mentioned, there isn't any aiming lag in KeyMander but if you exceed Rainbow Six's maximum turn speed you will feel it move slower and slower the more you exceed the limit (the faster you go). Basically, while you are moving faster than the game's max turn speed input, the game will stop responding until the speed drops back down below the maximum input level, giving you that "lag" feel. R6S has a much lower maximum turn speed than games like Call of Duty Black Ops 3, so the effect is much more noticeable. To get familiar with the game's maximum turn speed, just hold the right controller stick all the way to the right and watch your view to see the fastest look speed that game can attain. You will need to get a feel for this speed and learn to move your mouse within this speed limit. You also need to set your mouse properly as described above, since too much boost in sensitivity will easily cause you to jump above the max turn speed and feel.

Here's our sample profile for Rainbow Six: Siege. After you download it and import it into the KeyMander software, remember to upload to KeyMander before you are done! Set the in-game settings as noted on the profile page and you should be good.