The investigation has taken very little time as yet relative to other investigations of sitting presidents (chart is ~4 months old, but we still have only possibly reached the end of the next shortest investigation) [0]

[0] https://fivethirtyeight.com/features/why-manaforts-flip-may-...

Mueller's been pretty good about getting evidence put into public record via the court system.

Led to believe by whom? People who have a vested interest in people believing it would be a short investigation.