on vocabulary explosions

As we aaaaall know, toddlers are linguistic geniuses who learn like 3000 new words every day whereas adults are veritable cretins who can't string two words together in another language to order a cup of coffee. (There were extenuating circumstances; I didn't know it was culturally weird to order a takeaway coffee in Italy; it all went downhill from there.)

Thing is, I don't think this is entirely true. Okay, yeah, most kids have the kind of capacity to pick up language subconsciously that makes the slow, laborous task of adult language learning seem, well, slow and laborous - except that actually I think we get a few breaks along the way.

I've been thinking about how much easier (easier? really? am I allowed to say that? it's a relative term; it's staying) it is for me to pick up Welsh vocabulary now than when I first started learning the language.

(People like to complain about Welsh mutations (treigladau), and sure, they're fun, but I just wish every word in the Welsh dictionary didn't start with a 'c'.)

As a totally unscientific observation, the more vocabulary I learn, the quicker I pick up new vocabulary. I have a few ideas why this might be so:

• Words are related. When you can start to pull words apart - with prefixes and suffixes and the like - and you can see how those parts are put back together, suddenly it's not so hard to remember that anghyfreithlon means 'illegal' when you've seen the 'freith' bit in other words relating to law, and the 'ang' bit in other negative words, and the 'lon' bit in adjectives or whatever they're called
• Improved memory capacity/capability. Just throwing it out there. Right? I'm sure you improve your ability to remember things the more you use it. I read that somewhere, I just can't remember where. (ha ha.) But seriously, they (whoever "they" are) talk about neuroplasticity as something that continues throughout your lifetime and is not just the magical elixir of screaming three year olds so probably right now I am forcing my brain to make new neural pathways that are better able to retain new vocabulary - albeit at a slower rate than when I was a kid.
• More accessible grammatical structures within which to put your new vocab. Legit. What I mean is that through practising other language stuff, like grammatical structure, I get better at those things which means that when I'm speaking Welsh, I use less mental resources on trying to put a sentence together which frees up some room in my brain to dig up some new vocab.

Is adult vocabulary explosion a real phenomenon? If so, what other reasons might there be for it? How do we optimise it and stuff?