MHz level stuff works for 'work' that normal people do - writing documents, emailing, etc. I dreamed for a while about creating a MHz-level processor from the ground up with an open hardware specification and creating a simple kernel + OS to run on top of it, with new secure protocols for networking (websites, not reinventing TCP/IP) and communications (email). Right now every single computer is backdoored by the governments of the most powerful countries: we have Intel MEs and AMD PSPs in the hardware, and the frameworks everything uses - Linux, OpenSSL, etc. - are almost all huge complex codebases that the NSA has tons of zero days for. If you could get open MHz-level hardware and get OpenBSD or something to run on it, and develop applications with a focus on secure code, it would be pretty great.

But ultimately this would all fail. Aside from the network effects - nobody's going to use it - the fast processors and tons of memory are necessary. We just don't think about it unless we're running out of it.

- high resolution photo and video - photo editing programs like GIMP and Darktable can spend some time processing photos; these days even with complex effects you usually never experience lag more than a few hundred ms on many megapixel photos, because our hardware is fast. Same for video, the memory and storage space and bandwith is a hard requirement, and going back to 360p is not really acceptable.

- high resolution monitors - no point in having great looking 4k 10 bit color video without a 4k monitor, and now you're stuck having to push 20gbps through your displayport cable on a 500 MHz processor. And text also looks much better at 4k. Also you could say this is a bit unnecessary but compositing window managers are pretty great and I would say a core requirement of modern GUIs, and having eye candy like wobbly windows, window shadows (those are actually very helpful, try turning them off), etc. is expensive. My Dell XPS laptop from 2017 couldn't handle wobbly windows without visible stutter at 4k60; my desktop with a $400 GPU from 2019 can keep up at 4k@144hz.

- new technologies - VR/AR, fast voice recognition and neural networks - this is all cutting edge stuff but the use cases are obvious and they have started to be applied more commonly. Also the failures like eye tracking, Kinect - they may have failed commercially but they were good ideas and a valid use of fast computers. Also online meetings with many participants, each with their own video streams that require decompression.

- obviously, video games - not that you can't have fun games with shitty graphics, but good graphics are nice.

I thought of a few more but can't remember them at the moment. Oh, also, Bluetooth audio - 192kbps audio, but you need to decode it since it takes compression to get it to that level, and then you have the additional overhead of sending it over a digital protocol instead of just having your ADC do the work. You would need expansion cards or a dedicated audio core in your CPU to accomplish this if your computer wasn't fast.

And of course, science needs big data and big hardware to process all of that data.

If you're only working with text then slow computers are fine, but as soon as media gets involved they are not practical.