First off let me say I am NOT a mortgage professional, so I do not know all the ins and outs of the 4506T Request for Transcript of Tax Records form and how it is used, so if there are specific questions check with your lender.
But here are my thoughts from several recent experiences.
You will generally be asked to provide copies of the last 2 years of taxes as part of your mortgage application. The 4506T is used to request a FORMAL copy of the tax records on file at the IRS, to make sure these are the SAME as the copies you have provided, all of which is part of the efforts to prevent mortgage fraud.
Once the 4506T form is sent off to the IRS, often through a 3rd party vendor and not your lender, the records typically take 48 – 72 hours to be returned, so timing is critical if you are waiting on a loan approval. You ARE at the mercy of the IRS and there does not seem to be any way to get them to move faster.
Make sure the form you sign is VERY CLEAN and easy to read, specially the signatures, as it will be returned by the IRS if they are not comfortable with the quality which will delay your forms even further (an experience I just had, twice, with buyers, causing a major delay).
In case you missed them, here are the previous REALSTATE-ESE posts: