While that would indeed be awesome, that's not the route they proposed. It's more about slowing down the perception of time, rather than being able to actually do something peoductive during that.

Philosopher Rebecca Roache, who leads a team of scholars, explains two methods to this madness. The first involves psychotic drugs that distort a person’s sense of time.

With a simple pill or injection, prisoners may believe they’ve been incarcerated for much longer than any natural human life could allow.

The second approach Roach explains is a bit more complex. Option number two involves uploading human minds to computers (da f*ck?), and speeding up the rate at which the brain functions. On her blog, Roach writes: "[...] This would, obviously, be much cheaper for the taxpayer than extending criminals’ lifespans to enable them to serve 1,000 years in real time.”
https://goodmenproject.com/featured-content/new-technology-could-make-inmates-feel-like-theyre-serving-a-1000-year-sentence-in-8-hours-scrol/

Despite thinking, "wow that's a disgusting way to see and treat humans", and some obvious moral concerns (like, social isolation for what feels like 1000 years, which will fuck up most people badly), which make this feel like a black mirror episode, the mind-upload issue is technically extremely tricky. Even if we had the technology to "upload" the human mind, it will be a copy, a clone, not you individually. And if we don't have an option to download the copy back into your brain, it will just be a waste of energy.

More importantly, an intriguing question is raised: After such a download, will this be you? Or just a copy of a copy and thereby another being which just replaces another one.

Another thing I find important to ask here: what's the point of penalties? These suggestions seem to me like psychological torture rather than measures to "correct" social behaviour. In no way resocialisation seems to matter here. So we just fuck people up by that and unleash them onto society afterwards. Doesn't sound good to me.

Sorry for not keeping my reply focused on your idea. I had some time to spare and this kept me busy.