That's only ~187MP/s. A single 4k 2160p60 stream needs almost 500MP/s, which might be at the edge of what's possible on a CPU today, but would make far more sense for a GPU to do.
It shouldn't be a stretch for a quad-core desktop processor today. Doubling the core count and increasing clock speed by 40% compared to a mobile Core 2 isn't hard. DDR4 instead of DDR2 means memory bandwidth is probably not an issue, and AVX can probably provide further headroom on the compute power.
And, of course, it's much easier to build a desktop with far more than four CPU cores these days.
I wrote a multi stream mjpeg decoder some years ago and it could run three MJPEG streams at 1920x1080@30 on a 2008 Macbook Pro (Core 2 Duo, 2.5ghz ).