There is a timeout on multi-key mappings, so if you need to type fd0 you type f, wait 'timeoutlen' (1 second by default apparently), then type d0.
That's a pain, but it doesn't come up often. In return you get quite a lot of functionality (due to vim being modal) without having to move your fingers beyond the letter keys, which is quite relaxing.
What if you need to type `jj`? Not uncommon to do so while writing math or physics papers. a_{jj}