How does that translate to smart contracts / dapps?
Performance of a dApp is the performance of Ethereum network.
When crypto kitties was launched, users went into a frenzy and started buying and selling so many kitties that it clogged the entire network.
In such a scenario every other dApp and transaction was affected adversely. Gas and transaction time went high.
In an unregulated network like Ethereum (only regulation is gas limit per block), one dApp can produce any number of transactions depending on user activity. Ideally networks are smooth but if a hyped ICO is active or dApp is in a frenzy you should expect clogging of the network.
Very fast , no issues since state tiers are maintained on node, you can read data instantly. Reads don't care about the network state.
Are all computational resources added up because all nodes execute the
No, every node is a copy of the other node. Miners mine the transactions broadcasted to one node. Node is a peer which shares information it has with other nodes. Nodes maintain the state of the network. The computing of transaction are done on the mining machines.