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Decent folding bike frames are really tough, so you don't need to really worry about it breaking in half underneath you. The small wheels do make a big difference to handling though, my 20-inch dahon feels a lot twitchier than my 700C road bike, but at low speeds it's much more nimble for picking your way through crowds. It takes more effort to maintain a high speed though, and when you stop pedalling you find yourself slowing down quite a bit quicker than you would on a full-size bike. That bike is even smaller, so these differences will be even more pronounced.
Folding bikes have their place, but you need to accept that they don't do some things quite as well as their full-sized counterparts.