The problem is that the market moves every day, and until you lock-in a rate with the lender they cannot determine what the fees and costs will be. And you can't lock-in a rate until you have a specific property identified and have submitted an application for the loan.You won't have a time crunch once you have a contract; any lender can give you rates, terms, and costs in just a few minutes when you have the specifics of how much you want to borrow. The long time occurs after you accept a deal with a lender and the underwriting kicks in, along with having to wait for outside sources to get back with info (surveys, appraisals, verifications, etc.).
You cannot negotiate mortgage terms until you are ready to sign a loan application and have a signed home purchase agreement.Read "Home Buying for Dummies" or any of the dozens of self-help books written for first-time buyers. If you go into this without educating yourself, you are VERY likely to make serious errors costing tens of thousand of dollars.