In my early days as a computer tech (around 2001), I remember doing some work on some clients' then-outdated AVID Media Composer systems, from around 1993. Those used an early Power Mac 8100 system (which had 3 NuBus slots), added a NuBus expansion chassis (another 3-6 slots I think), and a SCSI RAID array. The Power Mac's slots held the interface to the expansion chassis plus two custom AVID boards, and the expansion chassis was full of AVID boards, too.
In essence, the Power Mac simply drew the user interface and marshalled the AVID cards to do the work amongst themselves; the CPU was orders of magnitude too slow to do what Media Composer did. (Pretty much the only thing done on the CPU was certain effects rendering, and it took foreeeeeeverrrrrr…)
Over the years, AVID systems were able to gradually reduce, and ultimately eliminate, the custom hardware as the CPUs and GPUs became powerful enough to take over — for SD. When uncompressed HD came along, the whole cycle of custom hardware and a gradual shift away started over! (Now any decent computer with a good SSD can manage 4K video, provided you've added the appropriate interface card to get the video in and out of the system.)
It is incredible that we can now shoot and edit 4K video on a phone. It's a testament above all to the progress in GPUs. CPUs would choke trying to do realtime effects at those resolutions.