The difference is small if you use mini.iso (net install) and a minimal desktop version but it can be noticeable depending on the kernel version, packages that you add, 32 or 64 bit capability, available ram, other hw components etc. Debian is faster though, and free from the usual ubuntu bloat, install backports, correct drivers and put i3wm or awesome on it and you'll get a usable machine even if it's very old. Not sure if it would play nice with mac hardware but you could always try.
It is. It also feels unfinished, but it really depends on your expectations.
Personally, once I have a functional terminal and a web browser, I am ok, and lubuntu is perfect for this kind of use.
Agree. I develop on Lubuntu using IntelliJ and it works very well for that purpose.
Lubuntu is really fast. You might want to try Debian as per another comment, too; this article sums it up nicely: