Hmm that doesn't sound right about the escort mission. I play on normal and I got a few minutes to train some firebats/marines/medics and I have them follow the tank while I build more troops. I even got to build a few bunkers.
Are you using any cheats at all when trying those missions? Cos maybe the resources might be trigging the event to start earlier?
The funny thing is, when i use the trainer, i have no problems, i can have bunkers and all that made before the first transport is moving. lol
But when i am not using any trainer or cheats what so ever, i have around 5-10 seconds before the starts moving.