MySQL slow_log table: querying takes forever
Answer originally left as a comment by Michael - sqlbot:

Mostly it's a MySQL thing.  The slow query log "table" -- by default -- is actually just a CSV file read by the "CSV storage engine," which emulates a table but uses a simple CSV file as its tablespace.

This allows appending to the slow query "table" to be a low overhead operation, but of course a CSV file has no indexes, so it has to be scanned from the beginning.

I haven't used Google Cloud SQL so I can't offer an authoritative answer on what options you might have, there, such as changing it to a MyISAM table and indexing it.

See Selecting General Query and Slow Query Log Output Destinations in the product documentation.