Lack of optimization. Somebody has done a test with the GS5 in gaming and he used an on screen CPU and GPU monitor and CPU/GPU usage is always sitting at 40% for most games and CPU/GPU never clock up to max speed.
His youtube channel is Chris Nacca
HTC Incredible S S710e (Eventually stopped turning on)
Nexus 7 2012 (Half the screen stopped responding to touch after leaving it for 2 months, also has weird marks on bezel, no idea from where)
Sony Xperia Z C6603 (Passed it down to family member, I actually hated this device)
Samsung Galaxy S5 SM-G900I