Last time I checked, and feel to correct me if I am wrong, MySQL used Row lookups when doing JOINs. There was no support for more efficient join algorithms like hash-join or merge-join, something which kills performance if you intend to run JOINs on a result/working-set bigger than just a few rows.
Are there any improvements in this area, or is it still just the same old, same old?
Now older versions of MySQL, like 3.23, are still in use (unfortunately) and pretty crappy but it was released over 8 years ago. Most of these performance myths about MySQL are outdated.