Why is count(*) slow, when explain knows the answer?
Explain is using previously gathered statistics (used by the query optimizer). Doing a select count(*) reads EVERY data block.
Here's a cheap way to get an estimated row count:
SELECT table_rows
FROM information_schema.tables
WHERE table_name='planner_event';

Even if you did select count(id), it might still take a very long time, unless you have a secondary index on id (also assuming id is a PRIMARY KEY). Because all data (including Row Data) is stored in B-Tree indexes, performing a select count(PK_COLUMN) is still a considerable amount of IO (needs to reads all data pages).  If you have a secondary index on the PK field, it will be able to perform less IO to perform a count.