Update: Turns out the S6 was being limited by Touchwiz Android which is Samsung's version of android so that's why I had to have higher clock speeds in game as well as other root tweaks.
What I found out was that S6 had so much hidden potential that got released by the Android Stock ROM (also known as AOSP) which is essentially pure android. It meant I could run at lowest clock speed of 800mhz on all 4 cores big and something low like 1.0ghz or 800mhz or even less on the little cores. The game was easily playable even at 150% packsize, super smooth and no zombies that way. That was just on battery saver mode.
As far as my S5. Oh boy I have a lot to tell you. The S5 is a nice phone but what's limiting it in dungeon quest is Touchwiz android , Hotplugging (turning cores off automatically and limiting IPC so the amount you can get done at lower clock speeds) as well as default GPU governor. The GPU really struggled and also the CPU at stock setting with or without packsize.
Rooted the S5, turn off hotplugging , changing GPU governor to sync with clock speed or performance to squeeze most juice out of it and its optional whether you want to keep Samsung Touchwiz custom ROM or Touchwiz alone or Pure android custom rom.
I found that I could run DQ fairly easily at 1.4ghz or something low on 4 cors CPU on the S5 which is an old CPU 32 bit but its not that much weaker than the S6. The GPU however was like much weaker than the S6 GPU but even then, I managed dual core 1.2 or 1.4ghz , high fx or max settings , super high packsize number,high shadows or low shadows and well not only speedy gaming performance that I also felt on urooted S6, and saving a ton of power and battery.
I didn't even have to undervolt or over clock the phones once I used Custom ROM pure android and easily achieve 6 hour battery life or 4-5 hours when gaming. Maybe even way more. S5 battery larger which I like. Turns out I can get anything to run DQ nicely. Problem is that if DQ slows , physics and everything slows and that can cause zombie glitches.
If I wanted to however , I could run something like a really low clock speed and lowest settings for ultimate battery saving while still having the game fast. I could have changed the resolution of the device but I didn't have to. Major difference between S5 and S6 is GPU and storage speed being doubled the S5s. 385MB/s rather than 220MB/s. No regrets rooting and having fantastic devices.
I had the S5 used for really good price and I don't regret it. The S6 of mine did have cracked screen and water damage and I decided new phone is best since its very expensive to repair the phone screen . I could've bought the OnePlus 3T since that's an amazing phone even out of the box unrooted if I spent more or even a tablet that is very powerful.