How did the Hindu Arabic multiply?

Answer 1

You now add the results from the right hand column leaving out those were the number on the left is even i.e. rows 2, 4 and 5.

In other words, it's just the shift-and-add algorithm: .

...which can really be done in any number base (and is even "easier" in base 2), so I'm still not convinced.

All we need to carry out the multiplication is the ability to multiply and divide by two! Somewhat simpler than the same operation in the Hindu-Arabic number system!

Maybe it is "simpler" but certainly not faster, because it glosses over the fact that division by 2 of Roman numerals is itself not exactly straightforward.

Roman numerals are a mix of positional and unary-ish "repeated symbol" --- that's what makes them hard to manipulate.