Actually you can. Overclocked my S5 to 2.8ghz . s6 I did to 2.4ghz big cores and 1.8ghz little cores.
But man let me tell you, S5 at 2.8ghz is so fast! Feels like modern flagships but super responsive.
But 2.8ghz let alone 2ghz is overkill for DQ on the S5 and S6 IMO on my custom ROM. I play DQ at 1.2ghz at ultra settings and stuff very easily but for 60fps , 1.5 or 1.7ghz. On the S6, I managed 800mhz just for DQ which is the lowest frequency on its big cores and even that was overkill for DQ at ultra settings for me somehow.
How? Custom kernel and custom ROM. Also rooting the phone. OEM locks are irrelevant . if you talk about bootloaders such as locked phones by Verizon, T mobile and that. Well I use unlocked phones.
One thing I find is that Samsung versions of android limit you and their default settings but pure android is where I gain much more performance as well as battery life and I feel the full power I never would've otherwise felt.
But you can unlock bootloaders on locked phones but sometimes its difficult depending on carriers.
I could go into more detail but that's better off when I private message you as this forums meant for DQ rather than phones.