First point is CAP theorem is for distributed databases and should be read in those terms.
Now coming to CAP theorem, as per Wikipedia:
So when you say MySql default behaviour is master-slave, then it means master and slave nodes are just primary and secondary nodes and at a time only single node i.e. master is being used to process any user request. Hence, Partition Tolerance is already compromised here.
But the database is still Consistent, because all request (read/write) will go to the master node and user will get same consistent data all the time.
And the database is Available, because all request will receive response as all go to same Master node.
Hence by default Mysql in CA database. Slave lag couldn't impact either consistency or availability here.
There is another possibility for MySql, where via some configuration tweaks it can be made CP database and availability needs to be compromised. This will be kind of multiple master nodes distributed and remain in sync via networking between them. In that case, if any of the network or partition breaks among them, then Consitency is preferred means all read request will get same data, no corruption because Availability for write request is compromised.
