At the time I took them (I took 11 in the early 90s, mostly all exam no coursework), I remember arguing that all the actual content could actually easily be learned in sixth months, rather than two years.
Some of my GCSEs I could easily pass well now with little effort other than a quick scoot through the topics and content over a few days or so (Eng lit, Eng Lang, French, History, RE). Others I’d need maybe 3-4 weeks just to review and relearn the material comfortably (Chemistry, Physics, Biology, Music). Finally, the ones I would have to go back through in a lot of detail are Maths and Spanish, both of which I would need a good 3 months plus to relearn, I reckon. Even though I did maths A-level!
I’m an academic, so my skills in terms of absorbing and synthesising academic material very quickly are much better than when I was fifteen, and I use those skills every day, plus I can write much better and more quickly. I think those skills would easily compensate for the lack of youthful mental energy teenagers have. I also keep my French up to date to beyond GCSE level, so I don’t think I’d have a problem with that.
In the sciences I’d need more time to re-learn the content, even though I did some science A-levels. In music I could do the music appreciation parts no problem, and I could sing for the performance element no problem, but I have forgotten most of my music theory and composition so I’d need a bit of time to re-learn those!
I have let my Spanish get very rusty so I would need 3/4 months crash course in that. And I would need to really go through all the maths again from key stage 3 upwards, to be honest. 😆