Absolutely true. In an urban environment that sonsists of 200 meter sprints between traffic lights a 20" bike is pretty unbeatable due to its quick acceleration.

I also find my little bike climbs hills and mountain roads well.

It is however outclassed by 700c going downhill or long flat stretches as the larger diameter wheels carry momentum better and usually road bikes also just have more gear inches onboard (thanks again to their larger wheel diameter). I ride a Xootr Swift and find that I can tuck in and draft really fast roadies (even on aero bikes) over many flat Kms, but I cannot maintain being out in front at the highest speeds as the bike just isn't quite efficient enough to keep that up without someone breaking the wind.

​​​​​​20" wheels are also not ideal for rough or broken track. I ride it over such all the time because there's plenty of washed out dirt trails were I live and the bike does it alright, but larger wheels would be better and safer over uneven road surfaces.

Small wheelers are a lot of fun. They naturally accelerate quickly but over a certain speed more watts of energy are required to keep the wheels spinning. Considering their smaller storage profile I think that 20" is well worth considering if you want speed and the main ride environment is a city. And they're pretty good in many other environments too. All bikes are compromises.
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