Holding check proceeds superficially seems businesslike; however there are paperless mechanisms to transfer funds quickly without paying fees such as those involved in wire transfers.
My last check, which was sent by my bank's online bill payer, was not credited to my IB account for almost 4 weeks; that credit followed an "Trouble Ticket" initiated by me.
I also have several accounts with Fidelity, and their Moneyline works efficiently and without delay to transfer funds in either direction without a direct charge; I believe it is actually an ACH transfer. Even the federal government has a mechanism to accept ACH-like electronic transfers for quarterly and year end tax payments. Of course, one can transfer an entire account from one broker to another electronically.
Although I am a GREAT fan of IB, it is possible that playing the float on checks, cost shifting, and/or back office problems are additional factors in the delay.