Users are required to wait for a period of one week when moving assets out of Optimism into the Ethereum mainnet. This period of time is called the Challenge Period and serves to help secure the assets stored on an Optimistic Ethereum network. You can find more information about the Challenge Period here. Please note that the Optimistic Kovan testnet has a Challenge Period of only 60 seconds to simplify the development process.
There are third party bridges that allow you to withdraw your assets faster, typically by fronting you the ETH or tokens and then performing the a large withdrawal when they run short.