Why restore_command is called for existing files in pg_xlog?

Hello hackers,

There is one strange and awful thing I don't understand about
restore_command: it is always being called for every single WAL segment
postgres wants to apply (even if such segment already exists in pg_xlog)
until replica start streaming from the master.

If there is no restore_command in the recovery.conf - it perfectly works,
i.e. postgres replays existing wal segments and at some point connects to
the master and start streaming from it.

When recovery_conf is there, starting of a replica could become a real
problem, especially if restore_command is slow.

Is it possible to change this behavior somehow? First look into pg_xlog and
only if file is missing or "corrupted" call restore_command.

Regards,
---
Alexander Kukushkin