It sounds to me like you have quite a few misconceptions about this process. BIAB involves doing a FULL mash in the bag, this actually a step beyond what you would do with a partial mash.
I think you may be confusing “mashing” with “malting”. Malting is what the maltster does to enable the starches in the grain to be converted to sugars during the mashing process. Mashing is what brewers then do to convert the starches in the malted grain in order to produce sugars that we can extract as wort.
BIAB does take longer than an extract brew because you need to perform a mash for at least 60 minutes before pulling the grain bag and starting your boil. Also, if you are using Pilsner malt as one of your base grains, then your boil should be for about 90 minutes instead of the usual 60.
I don’t know if you were doing a partial boil for your extract brews, but you need to be able to boil the full volume of your wort when doing BIAB.
The BTV episode on BIAB is an OK starting point, except in real BIAB you don’t tie off the bag. You leave it open in your brew kettle so that your grain is fully mixed in the full volume of water during the mash. Hunt down some of the episodes on Basic Brewing Radio/Video regarding BIAB for some better examples.