Fastest...
Make sure motherboard supports SSD drives speeds of Samsung 970 EVO NVMe or equivalent.
Ram...honestly not too big of a deal (for BIAB) I did not notice ANY difference between 16GB and 32GB Ram installed speed-wise.

Donny mentioned soundcard (If he means a built in ones...)I will disagree. Most laptops, even higher tier will have your Realtek chips... Which pulls BIAB just fine with Asio4All drivers.
It is MUCH easier to get a good quality, basic 24bit external card under $100 that you might have already...than to look for laptop that has something specific "built in".

My personal advice, get a laptop with at least one USB-C v2 (AKA Thunderbolt 3) port. Preferably 2 of these. If you ever decide to go external/audiophile for BIAB and need ports to be "fast", that would be your fastest. And those wonderful ports support video signal too. So when I do my BIAB choirs, I have a second monitor connected to laptop (to Thunderbolt port) with my VST's on the secod monitor so I can adjust and see stuff without constantly opening and closing windows.