I queued for at least 45 minutes at Terminal 5 Heathrow for boarder control not too long ago. Also took me best part of an hour to get through security at London City earlier in the summer. The last two times I’ve flown through Stansted I’ve been pleasantly surprised with both security and boarder control. These issues are not specific to Stansted, however the key is the time at which you arrive. 2200-midnight is when all Ryanair based units arrive back at Stansted. I do my best to avoid arriving during this time.
As as for the blue car park, isn’t that the cheapest option of all short stay? Hence the walking distance. The new multi-storey is worth paying that bit extra for.