The device getting warm under heavy CPU load has nothing to do with the efficiency of the operating system. One should note that the CPU's between Symbian and WP8 devices are very very different, Symbian devices use the old ARM11 single core CPU's and the WP8 uses S4 dual cores. The processing power between the two are in a completely different leagues and the S4 will get hot when it's doing it's magic.
Originally Posted by rohan_is_switching
But still, you can consider WP8 a very efficient system by modern standards.