What is an example for a decidable language not in P?
Yes, there are decidable languages that are definitely not in&nbsp;P. The time hierarchy theorem says that P$\,\neq\,$EXP, so P$\,\neq\,$R, independently of the P vs NP problem. Any EXP-complete problem is definitely not in&nbsp;P: for example determining whether white has a winning strategy from a position in generalized chess ("generalized" in the sense of allowing a board of any dimensions, with any arrangement of any number of pieces, but otherwise following all the rules of standard chess).