Immigrants to any country are a unique slice of the population because they chose to migrate for a better life. Hence immigrants in most countries tend to do well economically (sometimes after a generation). Uncertain that cultural differences are the reason for different economic outcomes you observe.

Swedish speakers in Finland are not immigrants anymore than Dutch speakers in Belgium are, though (the Dutch speaking area also generally does better than the French speaking areas, for a variety of reasons).

Which immigrants? A significant fraction of native Finnish citizens speak Swedish as their primary language. It used to be the same country.

They are usually richer too.