Is it feasible to manually implement chroma keying?
No you wont be faster, but you could use a different method. As in a different application. Really it comes down to what your bottleneck is. If the bottleneck is the interplay between you and the computer then sure. What you can do is expand the video to frames as a prepass. This takes time but is essentially unsupervised. Likewise you could preprocess the mask somehow (use a half size proxy image for example), and use that as a basis.

The problem is that it might not be economically feasible. Since:


you need to learn a new software, as well as find it. Test it etc.
You may need to write portions of it yourself, atleast scripts.


You still dont get fast enough to get your investment back. I mean its still cheaper to buy new hardware, if you plan to do this more than 3 times in a row. I understand that your circumstances may be special and you havent got the money.

But on that note, we are actually doing what is essentially chroma keying on the graphics card of a raspberry Pi and it works at 15 frames per second on a full hd image. So i doubt its the hardware computation per see. Possibly its a IO problem.