It's not just Thetford events that you get a tailback at the first singletrack/obstacle…..
Virtually every race/event I have done has a slow section in the first lap – even the Wiggle Enduro 6 where they give you a thoroughly enjoyable 1/2 mile run at the start to try & break things up.
You're racing for 12hrs…..does a 10 min delay on the first lap really matter – I look at it as a good chance to have a steady, warm up first lap.
As for the braking/acceleration bumps – the trails are very undulating, but I wouldn't say they are traditional braking bumps, except for in a few places – mainly on the red/black route.
If it does rain at Thetford, generally the mud is very sloppy so doesn't clag the bike up – I have only done one race where the mud got claggy & it was basically drying out & all the bikes turned it into a gloopy mulch that stuck to everything.
You'll have great fun! Tearing through the singletrack at Thetford with lights blazing is fantastic.