If you are walking from south into the arena, as arena only, it takes 10-15 minutes to get into the arena. Will be slightly quicker this year too, as you enter the arena further up now.
Hellfest have tried to get MCR for the last couple of years, so I wouldn't be surprised if it is Slayer, System, Gojira and MCR for next year at Hellfest.
I can see Download as MCR, System and one of Blink, Nickelback or NIN as the third headliner with Gojira subbing System or headlining second and potentially Slayer headlining second (but I think they will more likely not play Download because of the cost unless they want to play it).
My understanding is that the queues returning back to the UK will be much worse (up to two hours) but the queue entering Lisbon isn’t so bad. That’s just from reading a few forum posts about it though so take it with a pinch of salt!