I think the "speed" is because this change probably just shows up in Google by virtue of changes propagating in the GNIS feed [1]. It showed up in the feed Aug 27th with a note on the executive order [2].

[1] https://www.usgs.gov/tools/geographic-names-information-syst...

[2] 'Secretary's Order 3453 directs the Board on Geographic Names ("BGN") to immediately rename Lake Ontario to Lake America and to update the Geographic Names Information System to appropriately reflect the change. This Order implements President Trump's direction in Executive Order 14420 "Honoring the American History of the Great Lakes and Renaming Lake Ontario as Lake America," that the name be changed. The variant name, "Kaniatarí:io Lake", is a Mohawk name which translates to: "nice lake".'