Yup, force GPU rendering makes it worse for me to play DQ in my experience. Also I kept having crashes but uninstalled and reinstalled and they disappeared the crashes. As far as zombies, still very few zombies and good framerate without GPU rendering enabled.
I often play the game with external music player as well which could be a big effect on potential performance but not much. Even at 1.4ghz dual core with 4 Extra little cores it runs fine (I lowered maximum clock speed) and the undervolt especially helps too.
Well the game runs fantastic now regardless and that's even better with the shadow update to reduce the lighting effects. I haven't yet updated to the shadow turn off fix as I'm on 4G ATM but yeah I will. Although I'm kinda nervous lol since I always run ultra low shadows and low fx . GPU wise it doesn't make my device as hot as games like Eternium and better so when undervolt and disabled dvfs. Farming feels quick for sure now and will probably be quicker if I choose a less intensive farm build (aftermath and packsize is intensive and with crushing flames).
Also I undervolt much further to -60mv or -75mv CPU and -37.5mv on GPU and -25mv on memory. Seems to help a lot for me and 720p is still the smoothest spot for gameplay for me. 1080p and higher doesn't feel worth it in performance to looks and more battery drain but that's up to you.
Lastly, I felt like the crashes stopped and improved performance since the low ram device Xposed module was disabled which means there's no limited on how much RAM DQ can use. Yeah ram does affect the performance of DQ if it can't use anymore or if your ram is too low. Now it can use as much ram as it wants and smooth experience. 720p is a good one because it uses less ram overall (the ram in your phone also accounts for Vram from the graphics as well as other things). If phones had separate Vram like pcs, then ram wouldn't cool as quickly and graphic performance will be fine.
That was the case for my S3 mini when it has low ram which meant I couldn't play as fast as I can on the S6 , although other factors such as inferior GPU, CPU and storage did come into it.
Still this post still stands as it works well for me.