Regular everyday federal investigations that involve commonly prosecuted crimes with defendants who aren’t political land mines can take up to five years. For an investigation into a former president and possible current presidential candidate who is charged with crimes that have almost zero legal precedent to be moving to trial in three years is astoundingly fast.

https://thetampacriminallawyer.com/how-long-can-a-federal-investigation-last/

“A Federal investigation can last upwards of 5 years due to most Federal Statute of Limitations prohibiting the Government from charging or indicting someone after that time period. It is not unusual to see an indictment that lists dates of offenses 3-5 years prior to an arrest.”