I will be applying for PhD positions starting in fall 2016. Most of the application deadlines are in January. If I send my application now, will it be processed immediately? Or are the recruiters waiting until the deadline has passed? This probably varies somewhat per geographic region (and university, and department) but I would be mostly interested in UK institutions in the STEM fields.
In my experience (in a large public American computer science department), review of applications begins as soon as they are submitted, but no decisions are announced until well after the posted deadline. Reviewing graduate applications actually takes significant time, and admissions decisions are based at least in part on comparisons with the rest of the applicant pool.
My university actually forbids announcing admissions decisions before the application deadline. Other universities/countries may have more relaxed policies.
It varies. Last year I applied early, and one (top 20) US school that doesn't do rolling admissions contacted me before the posted deadline to admit me. Then again, other schools explicitly wrote on their website that they don't look at applications until the deadline. Others said they "encouraged" early submission but didn't elaborate what that meant.
I don't think someone would be at a disadvantage for applying late. However, there may be some benefit to applying early. Maybe.
"Deadline" is a guide and is definitely not set in stone. They could admit you immediately or almost before the deadline to signup for classes. I doubt they'd do it immediately because it takes quite some time, people are lazy, holidays, weekends etc... They'd even do it after the deadline if you're extremely talented. There is no telling.... Good luck!