The IRS is reviewing your return and checking for accuracy before issuing a refund. The wait time is established by the IRS through the power given to them by Congress.
The April 15 filing deadline is a "fixed" date, though many taxpayers file for an "extension", which allows a later date for filing.
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***