Some more information... First of all I found out that marking PL/pgSQL function as immutable significantly increase speed of its execution: 19808 ms vs. 27594. It happens because exec_eval_simple_expr is taken snapshot if function is volatile (default). I wonder if PL/pgSQL compiler can detect that evaluated expression itself is actually immutable and there is no need to take snapshot for each invocation of this function. Also I have tried yet another PL language - JavaScript, which is now new outsider, despite to the fact that v8 JIT compiler is very good.
I have a plan to do some work in this direction. Snapshot is not necessary for almost buildin functions. If expr calls only buildin functions, then probably can be called without snapshot and without any work with plan cache.