Another person responded, but I want to give another, simpler explanation. The order in placing a mobile goes as follows:
1) choose time frame for mobile order (these are 30 minute windows)
2) place mobile order
3) walk to restaurant you placed mobile order at and be there within the 30 minute window you chose
4) tap "I'm here, prepare my order"
5) mobile order is prepared
6) mobile order is given to you
This is why your order will not be cold: it's not prepared until you tell them you are there to pick it up. This is different than placing the order. As others mentioned, step 4 some will tap it before they actually reach the mobile pick up area as a few places take additional time. I choose to wait until I'm right at the mobile pick up area. Personal preference.
The 30 minute window you choose is the time to be at the restaurant and tell them "I'm here, prepare my order". The window has nothing to do with picking up your food after you have told them you're there and ready to pick up.
Make more sense?