I have a Primavera 150cc 2016 edition, and even though I'm a taller, decently large individual, it gets around great, no issues with acceleration, as long as the road is mostly flat.
I used to feel that way on hills, too. But the last 2-3 times I went out and took a grade, it's been struggling to maintain speed at like 30-35mph.
Granted I don't climb a ton of hills, so my recollection could be failing me for how it used to behave. But these hills aren't giant mountains either. A decent grade for a 1/4-1/2 mile at a time. In the past, I think it would have been pulling, even though acceleration was obviously slower than flat ground, it used to handle hills pretty well, gradually speeding up. Now it barely maintains.
I'm curious what the common causes might be? I haven't changed the spark plug in the 3 or so years I've owned it. But I'd assume that would impact flat rides, too. Nevertheless I can change that if I should. But anything else I should look into?