"Really an old processor"... was afraid so - I'll just need to do something about that, I suppose. Meanwhile I followed up on some suggestions above and redid my trial render using GI instead of GR. Also, I used user settings, switched off all unnecessary options, and chose tile render. All else was left as before. The result (lower half of above image) finished in 1h54m! Less than a third of the time the GR render took. And IMHO it looks in fact BETTER than that - though it is somewhat darker, it is also distinctly less grainy.