I really want to use the cache API to speed up my slow builds, as I’m doing a lot of really expensive stuff like epub to mobi conversion; but I’m not about to use Jekyll 4 for external reasons.
Is there any cache backport or equivalent which I can use for 3.8?
No. The Cache API won’t be backported to Jekyll 3.8 and older.
Jekyll is open-sourced with MIT licence. So you’re free to write a plugin that backports the feature (and other optimizations introduced in Jekyll 4.0) for older versions.
Be nice, please. I just want to know whether anyone else has already done this to avoid wasted effort.
What they wrote was a bit dry but I don’t think it was mean. They answered your question.