how can I do screen video capture?

I don't believe there are any screen capture extensions, at least any that have been publicly released (someone correct me if I'm wrong please!). If your target is HTML5 you can use getDisplayMedia() and the screen capture api. If you're on Windows there may be a way to hook into Game Bar capture but it'll probably be fairly advanced to set up and your user will need a compatible graphics card.

Ah, nevermind, there is a screen capture object! It'll capture to BMP files but you can then convert the BMP sequence to AVI in the object, or you could even shell out to FFMPEG to convert it to MP4 or another format of your choice.

Compression is much more processor intensive than saving a BMP. Even saving a JPG is more processor intensive than BMP because it has to invoke a compression algorithm, versus a BMP which is basically full pixel data.

Uncompressed is definitely less CPU intensive than compressed. I think the professional screen recorders tend to use special GPU hardware encoding to perform compression fast enough - though I assume that's not something Fusion can do easily.

But uncompressed video is a lot of data, which will rapidly fill space on wherever it's being stored (HDD/SSD/RAM). If it actually has to save a BMP file each frame while it's recording (rather than storing it in RAM and writing after recording finishes), then I imagine that the writing to drive would become the bottleneck. Both in terms of time (how fast it can write and whether it can keep up with your framerate) and space. If you're recording full-screen, then you'll hit 4GB in minutes, if not seconds. If you use a method that saves a single AVI file, then going over the 4GB limit will cause headaches. If the method saves single-frame BMPs then you won't have the 4GB headaches, but you'll have a gazillion BMPs, which is probably its own headache. If you find a method that saves to RAM and writes to disk afterwards, then that's likely to be the fastest. But if (when!) the RAM maxes out (or even just hits the 32bit ~3.5GB limit), you're probably looking at a freeze or BSOD.

Depending on what your purpose and method are, you might be able to record only every second frame or so, which would probably ease performance (and filesize) considerably. Or automate it so that it saves smaller videos in separate chunks and flushes the RAM after each one.