They are quite random (inconsistent), that's the problem. I started the 1946 campaign and first noticed an immediately available design (stated as ready, not pending) around the 1970s. I used an earlier family (airframe?) researched earlier, but used different newer engines and fiddled with fuel load a bit. I designed a new turboprop, regional jet, business jet, and a special civil in one go, all based on previous models. And I had to actually research the regional jet only, the others appeared as ready, and I could build them immediately. I don't think it had to do anything with specific date, but rather a reused family, maybe some values didn't clear that were supposed to
Similar thing with the speed - it just started to appear at random. I remember designing a fighter when I first noticed this issue. I tried swapping engines, but it had little effect, the speeds changed all at the same time but not too much. This also happened around 1970s in the same campaign. I think it might be connected with the total number of designed planes (that gets bigger the longer you play); or the engines are not applied properly (some values are not cleared properly or overriden). I would also check is higher engine powers (in terms of numeric values) are at play here - it might be the 'too high number freaks the system' type of thing
Sorry for not giving you the exact reproduction steps, I just don't have them. The dreaded floating bugs...
And I'm pretty sure thrust and wing type do not affect maneuverability, only wing area and flaps do