This is just for the English speaker learning Spanish Duolingo course, which I’m told is one of the best ones, so it may not apply to other courses. But IMO it was easier to pick up the majority of the beginner vocabulary in Duolingo (they’ve got the drill aspect of language learning down pat) and then spread out to other sources. I especially needed outside help with grammar because (at least when I was doing the early parts) Duolingo didn’t explain grammar very much, so there was a bit of ramming my head against a brick wall.

How long an article takes me to read depends on how many colloquial phrases it has that Duolingo hasn’t introduced me to, if uncommon words or jargon are used, etc. The dictionary app I use is pretty good and includes slang, so when I do run into unknowns it only takes a few seconds to look it up. But overall I’d say I read maybe 1/2 to 2/3 the speed I read English, depending on all the above factors. It does fatigue me a lot faster than reading English, but I think that’s a normal thing for second languages you’re still learning.