And I thought exactly the same thing too, so I looked it up. The first thing is whether falamh 'empty' and falbh 'to leave' are related. The pronunciations are very similar and there is an obvious semantic link - if a field is 'left' then it is 'fallow/empty'. However, the mh / bh contrast is problematic, as is the fact that this verb is only used intransitively - I can 'leave' but the field cannot be 'left'.
Neither word has a confirmed etymology. MacBain simply lists some shots in the dark by other people, without being convinced by them.
As for English fallow, Wiktionary says
From Middle English falow, from Old English fealh (“fallow land”), from Proto-Germanic *falgō (compare Saterland Frisian falge, Dutch valg, German Felge), from Proto-Indo-European *polḱéh₂ (“arable land”) (compare Gaulish olca, Russian полоса́ (polosá)).
This chain of words is not too bad phonologically, but there is a glaring gap in the semantic sequence: how are we supposed to believe that a word that means 'land where you are growing crops' suddenly becomes 'land where you aren't growing crops'?
I think there are two factors. One is the belief by linguists in the past that 'it must come from somewhere' and, since they were infallible, they were prepared to accept the best they could find, even if it was fundamentally flawed.
And this forms a fatal combination with another belief. They were incapable of believing that a word in a sophisticated language like English could possibly come from an uneducated language like Gaelic. Well here's some news: in the past a huge proportion of the British population of farmers spoke a Celtic language.
If we are prepared to accept the possibility that a farming term went from Gaelic into English, then it makes perfect sense that a general term in Gaelic became an almost identical-sounding, identical-meaning term used by farmers in a specifically agricultural context. D
As far as we know, there is no separate slow speech in Gaelic Duolingo, or any other Duolingo that uses real voices. It is available in languages where Duolingo uses a computerized voice. Why they introduced the slow-speech button a couple of months ago is not obvious. If you click on a word in blue in the sentence on this page you get to see sentences that contain it. If you click on falamh then you get to see this sentence, and you get the option to hear it. But you can see clearly that there is only one speed.
If anyone knows of any sentences where there is a separate slow speech, please post.