Close out of all open apps
Even if you aren't currently using them, apps are technically running in the background on your device, and one of them may be causing certain lags elsewhere. Start with a clean slate by closing them out individually by double-tapping the home button and swiping each app upwards.
Do a quick reset
If closing out of apps doesn't fix the suckiness, shut down your phone and let it rest for a few minutes before restarting. If you're still seeing an issue, you can try a more aggressive option by resetting. Press and hold down the home and sleep button simultaneously until you see the Apple logo appear on the screen.