@Tragularius, good observation.
After 5. Bb3, white's B is pushed back but black might have a couple of issues to deal with.
First, he's not very developed. How many moves until he is able to castle kingside? And he probably doesn't want to castle queenside. So king might not be safe for awhile.
Second, black has to be careful with what is called a light square weakness. There's always the f7 square. A good target when the king isn't castled. But there's also the a8 - h1 diagonal which is opened with the black pawn push with white access from the d5 square.
Here a 10 year old Magnus sees how to take advantage of both issues for a relatively quick win:
Some months ago I was teaching the basic moves of some major openings to my brother, and when I showed him the Ruy Lopez (having played the moves 1. e4 e5 2. Nf3 Nc6 3. Bb5) he said something like, "Oh, seriously? Why would someone play like this, it's so boring/pointless, the bishop just gets kicked away and what have you accomplished?", making the moves 3. ...a6 4. Ba4 b5 5. Bb3. I knew this didn't make much sense, as if it did that would probably be the mainline of the Spanish, but I couldn't figure out why. Later I found out that this is called the Caro variation of the Ruy Lopez. In the database to which I have access (https://www.365chess.com), the defining move of this variation (4. ...b5) is the third most common move, having been played in only about 2.5% of the games from that position. Besides that, it has scored only 34%, contrasting with the 44% achieved by the two most popular moves (4. ...Nf6 and 4. ...d6, respectively). Thus I must conclude that this is an inferior line, and I would like to know the specific reasons why. Feel free to share your knowledge, analysis, experience, thoughts and also games!