Are all context-free and regular languages efficiently decidable?
Regular language membership can be decided in $\cal{O}(n)$ time by simulating the language's (minimal) DFA (which has been precomputed). 

Context free language membership can be decided in $\cal{O}(n^3)$ by the CYK Algorithm. 

There are decidable languages that are not in $\sf{P}$, such as those in $\sf{EXPTIME}\setminus \sf{P}$.