A suppression of economic activity would describe slow shrinkage of GDP.
India was always populous, so that was a constant.
You may also like to read about increasing famines in British controlled regions, how food crops were forcibly replaced with cash crops and so on. E.g. https://www.researchgate.net/publication/394628864_An_analys...
Unfortunately not many people understand or appreciate how colonialism destroys a country and they just see the end result and claim that the citizens of the country were incapable of developing by their own.
Not only economical, but also psychological (as in lose of self confidence) and cultural (assuming own culture is somehow inferior to the rulers) destruction happens. It is not much different from abused people losing self confidence and blaming themselves and additionallyhaving to suffer victim blaming from others - just at a nation scale.