The Ethereum cryptocurrency has a separate unit for transaction and smart contract processing fees. It's called GAS and it's a variable multiple of Wei - the smallest fraction of an ETH.
Laffer Curve is a concept from Economics, by which there's a curve for taxation and a curve for tax revenue. When taxation crosses a certain point, tax revenue drops because production gets limited by tax. In cryptocurrencies, when fees rise over a certain point, the number of transactions drops as well.