How h265 settings / presets affects the target device cpu while decoding and how can I tweak them?
Try Encoding using "--tune fastdecode" and also check if "Average Bitrate" with "2 Pass" is better than "Constant Quality" with your particular Decoder; trying to maintain Quality over Bitrate can spike the Bitrate (holding a particular Bitrate will drop the Quality on complex Scenes).

Try a few Test Files (short Clips from your Source) and determine which Encode Settings produce the lowest CPU load when Decoding. 

Decoding x265 works best with Hardware support which is not going to be available in most Phones, the Phone will probably decode x264 much easier since so many people/years have gone into decoding x264.

Reference: http://x265.readthedocs.io/en/default/presets.html

See also: http://www.techspot.com/article/1131-hevc-h256-enconding-playback/page4.html