Go to the nearest police station and ask them how long it takes to build a simple case like robbery.
Whether you like it or Not - to establish a PRIMA FACIE case (in contrast to a witch-hunt), you need to investigate, gather evidence and interview witnesses, before you charge someone. And that takes time.
Not sure where you guys went to law school, but its not as simple as a few TV interviews or a few transcripts.
This is NOT a one-hour Law an Order episode or some kind of reality TV.
A charge that the President had Abuse his Office, is a serious one and has serious consequences - ie removing the President from Office.
So its imperative the prosecutors, in this case, the House, build a solid case supported by corroborating evidence to prove beyond reasonable doubt - especially to the public.
