You need to give us the Carrier you are with, and the software version.
There is a lot of differences in the mix. Not all S5s perform equally well, especially if you were unfortunate enough to have gotten Lollipop from Verizon.
I'm still on 4.4.4 and my S5 performs fairly snappily. Not "instant", not even my desktop geewhiz computer is "instant"... it takes a bit of time to load stuff.
Once loaded though, you should be able to use Recent Tasks to return to where you were in an app "instantly".
Gallery is slower than other stuff, yup.
____________________________________
2 ea. Verizon Galaxy S5, 4.4.4 forever!, ART
MyPhoneExplorer lets you access a broken phone while locked
Nova Prime, Textra, Blue Mail, Qi wireless equipped
MacroDroid can help extend battery life
MacroDroid senses Screen off, turns Wifi & Data OFF
MacroDroid senses Screen on, turns WiFi & Data ON