Why would I trust a cryptocurrency that allow only 3 transactions/s?
You are equating the value of a currency with the performance of just one way to exchange that currency. There are poor ways to exchange dollars as well.

Also, I'm not sure what you mean by "invest in such service". You can invest in bitcoin-related businesses and you can invest in bitcoin, the currency. But I don't see how you can invest in the bitcoin generation and transfer service provided by the blockchain.