The actual computation in Bitcoin is cheap. The real challenge is spam prevention in a decentralized system. In bitcoin there is this rule that the longest chain wins. If a spammer can create 1000 blocks based on a chain that is 100 blocks behind the longest then he can "rewrite history" by making his chain win. So how do you prevent spam? Only allow a message every 10 minutes. How do you make sure nobody cheats? Force them to complete a cryptographic challenge that takes 10 minutes on average. Of course the side effect is that the more nodes you have, the more power has to be thrown away to maintain the 10 minutes timer. This is not a scaling problem though. You can just increase the block size to process more transactions or reduce the block time to have more frequent blocks.

Bitcoin is kinda like San Francisco. They decided to stick with the current block size and that made any use case other than buying bitcoin to speculate very expensive.