Of course, there are all kinds of products and homemade "recipes" for scum-removing cleaners but personally, I think one of the best and easiest things I have found are the Mr. Clean Magic Sponges (or the Walmart house brand works just as well too). Depending on how long it has been since the tile has been "de-scummed," it might still take some elbow grease but the sponges will eventually take most, if not all of it off. (The sponges also work quite well at removing rust/iron stains in the bathtub from well water.) I recently tried a recipe I saw here (or maybe on Pinterest) for a mixture of heated white vinegar, water and Dawn dish washing soap. I was curious to see if it worked any better than the sponges. I found it to be way too soapy for my liking and took a lot of rinsing and rinsing....and more rinsing. Little or no rinsing is required after using the magic sponges.
Easy Off Oven Cleaner No Fumes, with the blue cap. Spray it on, let it set anywhere from 5-30 minutes, either rinse off, or if it is a bit dirtier, than wipe it down before spraying it off. I swear it works. And to delay the deposits of hard water and grime, you can put a nice wax job on it with some Turtlewax.