Subaddresses are deterministic. Your wallet will only search ahead 200 subaddresses at a time in order to check for incoming funds at those addresses. Therefore it would not be recommended to attempt to discover 'vanity' subaddresses, because it would later be more complicated to restore your wallet from seed and instruct it to search for incoming funds at very high subaddress indices.
If you really want to do this, you can just keep clicking the button to create additional subaddresses in the GUI, or use the address new command repeatedly in the CLI. See instructions here for restoring your wallet in a way that will scan to higher subaddress indices: Funds received from subwallet are not showing
Setting up the wallet to scan for larger numbers of subaddresses is a computationally expensive one-time operation, after which blockchain scanning speed will not be increased. This means that if you're willing to tolerate the one-time setup costs, you can monitor huge numbers of subaddresses without slowing your computer down.