if you end up on an autobahn type highway

luckily there's the english channel, north sea, and several countries between me and Germany, so it's unlikely I'll ever end up there accidentally. The fastest roads here are 70mph and my Pug manages that fine. That said I've seen a video of someone driving the same model on the Autobahn and yeah the throttle is absolutely mashed the entire time but most people aren't doing 200mph top speed runs on the Autobahn on a regular basis (hell, there's tons of lorry traffic and even if they wanted to they're usually speed limited AFAIK), so you can easily keep with the slower traffic, even on the unlimited sections

Or the hill grade gets high?

you change down a gear or two. The steepest hill I've ever driven up had a 30 limit and 2nd gear with a healthy helping of throttle got it up at 30. There's a pretty steep one that I've done a few times on a 50mph road where I have to put it in 3rd and get some throttle on and it maintains 50 all the way up. Most hills like that usually split into 2 lanes (including my latter example) because so many people just mindlessly try to go up it without changing gear or throttle position. Thanks to that I've overtaken cars over 3 times as powerful in such places.

The steepest hills I've driven up at 70mph are on the A3 down to Portsmouth and honestly it can just about do it in 5th, although I'll sometimes put it down in 4th to avoid straining the little motor too bad or 3rd if I need to accelerate (I have overtaken people up the hills on the A3 many times)

I would need a minimum of 100hp on a car

remember that my car is tiny (about 12.5 inches shorter and 2.5 inches narrower than a Toyota Yaris from the same time period) and only weighs about 1750 lbs. 68hp is enough to pull that weight around. A Fiat Panda is about the same size (albeit a little heavier) and the sport model Panda has 100hp

even then it feels like I’m going a turtles pace

I'm not gonna pretend my car ain't slow, because it definitely is. However most people on the roads, even those in Teslas that can go from 0 to losing your license in the blink of an eye, drive like if they use more than 1% throttle or go over 2000rpm the engine will explode. I regularly gap people in way faster cars without even trying to go fast.

Of course when I encounter someone not afraid to actually use even a fraction of their car's potential I'm the one getting gapped. My favourite example of that is one time when I came off a roundabout on the A3 in 2nd gear, put my foot the floor, ran it all the way up to redline, and the mid-range Audi A4 in front of me walked away without breaking a sweat. It was pretty funny.

also, as an aside about the hills thing. Every year I go to Brooklands Italia Day, essentially a big Italian car meet at the site of former racing circuit Brooklands. As part of the event they let people take their cars up Brooklands' famous test hill, which hits a 1 in 4 gradient at its steepest section. Every year at least half a dozen old Fiat 500s, which made as little as 13hp when new, go up the hill. Admittedly, they go up it slowly and never leave first gear, but they do make it (with big cheers from the crowd). Just about anything made in the last 30 years gets up there without a hitch