This is one of those things that I searched for online, didn't come up with an answer, and so decided to try it, and write the results up for the greater good.
So, I recently decided to convert our digital movie library from MKV to MP4 for a number of reasons. Somewhat optimistically - and for comparison - I ran a couple of conversions using the Handbrake "HQ 720p30 Surround" defaults on my sleek but weak 2019 Macbook Air, but its mobile processor meant each movie took an hour and a half to convert, with lots of heat and a projected run-time of around 154 days to convert 600 movies (if I left it working flat-out 24/7).
Given that I want to use the Macbook in the next six months, and don't want it tethered to a desk/power supply the whole time, I abandoned that idea.
I started to look at powerhouse machines that would get the job done quicker, including HP's ProDesk 400 series, but as we're an Apple houshold, I settled on the Mac mini i7, which scores a blistering 24,000 on the Geekbench 64 bit tests.
I'm always weary of Geekbench numbers for real world usage though, so I started to dig around online to try and find out whether anyone had published Handbrake conversion times using the latest i7 mini, but I came up short.
As of today, I have data. Yesterday, at 6pm I added our library of 600 movies to Handbrake, selected the HQ 720p30 defaults, and started encoding.
Within the first minute, the fans on the mini notched up, the processor steadied on ~85% utilization and the first movie converted in just under 20 minutes.
This was encouraging, but I assumed thermals and throttling would probably kick in at some point along the way and slow down the the performance a little.
Today I checked in at 6pm, and the mighty mini had processed EXACTLY 72 movies in 24 hours - meaning it has consistently converted one movie on average every twenty minutes.
Using the mini over the Air (which I knew wasn't practical) shaved 150 days from the conversion time, bringing the entire task down to ~8 days.
In terms of overall performance, other tasks like opening Safari is just as quick during conversion as they were out of the box. So if you're in the market for a workhorse that's small, quiet and doesn't struggle with heavy processing, I can now heartily recommend the 2018 (i7) Mac mini.
If there are tests you'd carried out on the i7 Mac mini before you buy, drop me a DM on twitter. Software permitting I'd be happy to report back. But no, I'm not installing Windows on it...