How should I interpret the "lock time" within mysql slow query log?
The lock_time in the slow query log is actually the amount of time the query spent waiting to acquire the lock it needs to run. For example, UPDATES queries need a write lock.

The locking also depends on the storage engine you are using in the table. When writing, InnoDB will use row-level locking and only lock the rows that are being changed. MyISAM will lock the entire table until the update/insert/delete is complete.

The Locking of the entire table for MyISAM is a big reason SELECT queries will have a lock_time in your slow query log.