Slowest car I owned was my Citroën AX 1.5L diesel, 57 hp.
Including driver, codriver and camping gear, probably about 1300 kg, and I took it over the Alps. Topped out at about 60 kph in second gear up the hill (9% gradient average, probably more like 10+) of the Grossglockner.
The most annoying part was the unlimited autobahn, faster drivers would come up behind me, I'd move over, and they'd accelerate veeeery slowly and box me in behind a slower driver (or truck) and bleed off all my speed, and then all the cars I managed to overtake in the past half hour, would all blow past me again and then proceed to go 20-30 kph slower than my top speed.
In regular traffic the car was fine. Most drivers accelerate really slowly in regular traffic by the way, so even a slow car is more than fine if you don't expect to shift at 2k rpm in every gear.
The only real downside to having a slow car with a heavy load is people who don't speed up on the on-ramp. They do half the speed limit when they merge, and then you have two options: they either have a powerful automatic car and suddenly fly off as soon as they actually do hit the gas leaving you their spot, or they have a manual car and attempt to accelerate from near-idle rpm in the highest gear, in which case, even a slow car can get ahead of them by shifting between peak torque and peak power.
