Microchip's
https://www.microchip.com of course. Look up the 33F series (3.3V-only though). These can do 40 million instructions per second and this is a 16-bit core, the working regs are 16 bit and all the math ops work on 16-bit operands. There's also two 40-bit Accumulator with a set of accumulator-specific asm operations.
30F is a somewhat less powerful series which is also pretty power-hungry (electrical power), but it can run on 5v.
You can get expensive demo boards or just get an ICD2 and work with a chip. You can do audio work and control systems. Video, well you can generate text and display it on an NTSC TV set but you're not going to say play a video off a Firewire link to an NTSC TV set even without manipulating the image, that requires far more power.
MATLAB's not really doing a capture. Your hardware dongle or webcam or whatever is doing the capture. For MATLAB to display the picture is a start but hardly what MATLAB is designed for, MATLAB is about manipulating the data with algorithms not just moving it from a capture device to a display device.
An example of video DSP would be to sharpen a blurry image. There are a hundred different algorithms in this area. It would take a very powerful DSP to implement them in real time however and the code would need to be designed and optimized for that processor.
Microchip has a more powerful 32-bit core coming out soon, they've announced it but I don't see any orderable products at this time and no support yet.