This doesn't actually address why an Asian Tiger like Singapore was able to rapidly industrialise or the difficulties of implementing good governance and economic policies.
1) It's about 1600 years since Britain was a Roman colony. A lot has happened since then, such as the Dark Ages. When Rome was in charge the local population was in effect the Welsh (and Cornish and Bretons). How's Wales doing now relative to England?
2) Yes, the US was a British colony, but the dominant group in the US is still the descendants of the colonizers.
I don't the history of Singapore, but I would be curious to know who was in charge when England took over versus who is in charge now.
In all these cases there is a complex history which can't be glossed over by saying "they were all colonies". Important issues are the support they were given to create or rebuild institutions, population replacement, the extent to which they achieved control of their own resources, the institutions they were left with, or whether they were saddled with debt. Haiti was a colony that achieved its freedom, but was left with a crippling debt to its occupiers which it only finished paying off in the 20th century.
1600 years is obviously plenty. Should 56 be enough such as in the case of Gambia? I'm not sure. That's almost three generations.
How well did British colonisation work out for Native Americans? That would be a more fitting comparison to make. Look at life in the reservations.
Did the Romans meddle in British affairs after they were booted from the isles? How was life in the British isles in the aftermath (let's be generous and give them 100 years) of Roman colonisation for the common people?
Singapore is an interesting, and exceptional case. But also worth noting that even here, the economically and politically dominant Chinese Singaporeans supplanted indigenous Malays. The Chinese mostly arrived during the British colonial period.
People who espouse this sort of thinking also say similar things about South Africa. "Apartheid ended almost 30 years ago. You should be over it." Or that Black Americans - people who were only accorded full civil rights in the 60s - should get over centuries of domination since it's "already" been a couple of generations since they could vote and sit at the front on a bus. I could keep going.