[AnotherCat]

In case you have not tried this:

I have not tested it as we have no Win 7 machines left here now and while it works in Win 8 I have not got the slowness problem so cannot verify that it solves that particular problem or not. The settings I mention are from Win 8 but from memory I think they are the same, or similar, as in Win 7.

Leave the monitor at its native resolution (2560x1440) and at your 133% scaling setting so that it remains as you want it for other applications.
Navigate to the folder with your Calibre shortcut in and right click the shortcut.
In Properties>Compatability check the "Run this program in compatability mode" checkbox, select your current operating system i.e. Windows 7 (we are not trying to make it compatible with another version of Windows), and tick the "Disable display scaling on high DPI settings" checkbox (I am assuming that box exists for you).

Now when you start Calibre everything else including other running applications will remain at the display's native resolution and 133% scaling. However, Calibre will still be at the native resolution but will now be scaled at 100%. Hopefully, at that the slowness problem may be resolved but the text size in Calibre now needs to be fixed so it is readable.

If the speed problem is resolved, in Calibre go to Preferences>Change Calibre Behaviour>Look & Feel and change the Interface font to the style and increased size that suits you.

Dislocating the system's display scaling from Calibre's may give you the result you want of faster refresh speed. As I said, I cannot test this, so if you try it Good Luck.