Specifically ask them about lags and how do they happen.
Power saving mode also make the phone slightly less responsive and reduces refresh rate which some people interpret as lag, but it's actually normal with power saving mode on.
Several reasons cause lags, like what me and ihpwf12 talked about. Overheating also underclocks cpu and cause lags, though that is rare on S6.
Galaxy S6 has 14 nm chipset, where as the other flagships have 20 nm chipset. Lower nm is better. 14 nm chip is extremely power efficient and extremely powerful with least heating. This is why S6 has better battery timings (even with a smaller battery capacity) and performance than its rivals.
Go for S6, you will not regret it [emoji4].
Apple A9 chip in IPhone 6s and iPhone 6s plus is also 14 or 16 nm chip.
