Using "lo", the neuter article

[maromad]

Hi Laepelba:

The correct translation is: The good thing is how quickly I´m learning spanish.

Why?

1- "How quickly I´m learning spanish" works as a whole subject sentence here, it all is the subject for whom the verb and the rest of the sentence exists, modifying it actually. Try to put it the other way around: "Lo rápido que aprendo español es lo bueno". It perfectly works. You are emphizising that what you appreciate is the fact of learning quickly rather than learning spanish, which is what sounds like more on your own translation.
2- When you say "the good thing is learning spanish quickly" it also refers to something in general (the good thing when learning spanish is to do it quickly), whilst the original sentence means that as a result of a certain method(e.g) you are learning really quickly (at a certain moment or under certain circumstances)

Other way of saying it without losing the meaning and not using the pronoun "lo" instead is: The good thing is that I am learning spanish very quickly

I hope it was clarifying

Cheers