Why do they speed everything up?

It was just as "dumb" in 1979. The xeno grew from chestburster to full-size in like an hour of the movie universe's time (and just minutes of screen time).

This is reminding me of Prometheus, a movie that got slammed and bashed for stuff that Alien did as well, and stuff that was actually more "dumb" in Alien. I won't even get into Aliens.

Plenty of old classics have deep flaws and/or nitpicky things that are simply overlooked because nitpicking for criticism just wasn't as popular. I consider nitpicking to be a fun mental exercise, it doesn't apply to my enjoyment of a movie unless the movie is striving for real-world logic and fails in very large ways that affect the plot (and of course those kinds of flaws aren't actually nitpicks, they are logic fails, plot holes, character fails, etc).

These days nitpicking is often used as some kind of objective backing to a negative opinion, which is asinine.

This one has stumped my friend and I for over 35 years! Best we came up with was the lil booger must have feasted on Kane's innards and some how stored the nutrients to later be synthesized into a super nutrient by use of its silicone cell regeneration. Our speculation pretty much ended there as we had no fucking idea what silicone cells were nor the Internet to research. Now much older I know why...the movie would have sucked if not for the accelerated growth process...plain and simple LOL!