I have a g510, they are good devices, nice big 4.5" display, plays 85% of games nicely (even ones that "aren't compatible"). But you will need to unlock the bootloader and root your device before you can make it even faster than stock-standard.
This guide here will tell you everything you need about bootloader unlocking and rooting: http

/huaweinews.com/2014/01/root-huawei-g510/
Don't bother with flashing custom ROM's if your inexperienced, you'll cause yourself more problems.
I'd recommend this app once your finished the process: https

/play.google.com/store/apps/details?id=imoblife.toolbox.full, you can get your RAM usage down to about 210 if you utilize it correctly (uninstalling bloatware, cleaning junk).
Trust me, this phone is most definitely worth the money...