Maybe worth manually force closing any and all other Apps, to see if that helps?
Bear in mind that rebooting does not necessarily achieve same effect, iOS tends to reopen same Apps after a reboot.
In theory closing background Apps should not make any difference. Or at least, if it necessary (eg memory exhausted) iOS should eject them automatically, to provide more resources for the foreground App. In practice, I often find that when, say, Safari is getting inexplicably sluggish, closing other Apps most certainly does seem to help.