What you're experiencing is known as "mouse lag", and is kind one of those "classic" Nvidia problems. They have a really hard time with it, for some reason.
To increase performance there are a few things you can do. For one, turn off the shadows. That should help...a lot. Also, it sometimes helps to reduce the number of "prerendered frames" in the Nvidia settings.
How do I do that, you may ask? Well, let ol' digi show you!
First things first, you need to get to your Nvidia control panel. There are a couple of ways to do this:
1. Right click in an empty area of your desktop. You should see the "Nvidia Control Panel" in the context menu that appears.
IF that doesn't work, you can get there by accessing your computer's control panel.
2. Once you open the Nvidia control panel, you'll see a number of options to the left-hand side of the screen. The one that we're interested in, is under the "3d Settings" group, and is labeled "Manage 3d Settings"
In the main part of the window, select the "Program Settings" tab.
Now, there won't be an option to specify settings for SH5 specifically--we have to tell the Nvidia software that we want special settings for Silent Hunter. Sooo...click the "Add" button....
...navigate to your SH5 folder (should be c:\ubisoft\Silent Hunter 5), select the "sh5" executable in the window, and click "open."
Once you've done that, you'll see "sh5.exe" in the drop-down box next to the "Add" button, and a whole bunch of options below that. Scroll down until you see the item labeled "Maximum Prerendered Frames." Change the value from 3 to 1.
...and don't forget to hit "Apply" or your changes won't be saved.
Many users report a performance gain after doing this, especially when it pertains to mouse lag. Personally, I don't see much of a difference, but it makes the lag tolerable for me.
Essentially, Nvidia GPUs try to speed things up by having the CPU prerender a certain number of frames before displaying them on the monitor. It allows for smoother rendering most of the time, but, when a software cursor is involved, like in SH5, it can cause problems. This fix helps to alleviate that a bit.
Hope it works for ya. Good luck!