Creation of a screen magnification program c#/GDI -- need advice

Hello all. I have created a magnification program for a friend whose vision is quite poor. His problem with most available screen magnification programs is the fact that they only do whole-sized magnification increments (1x, 2x, etc.). Since there is a monumental difference on his 32" monitor between 1 and 2x, he wanted me to create a program with 0.1x increments. I did this by taking a screen capture of the whole desktop, cropping that bitmap to the size of the magnification window around his cursor, and then scaling the image by the scaling factor to his desired magnification. It works like a charm, but is painfully slow at high magnification/large window size (as would be expected). I was curious if anyone knew of any better ways/technologies to do this so I can improve the performance of my software. Regards, Justin