Eventually, I wind up with a query similar to:
UPDATE table_01 SET field_a = 'New value here', updated=CURDATE() WHERE primary_key=12345
Even though you've solved it one way to work out the problem here would be to change it to a select query (unfortunately mysql can't explain insert/update/delete statements - which would make it a lot easier).
ie change it to
select 1 from table where primary_key=12345;
If that's slow, then you're missing an index on primary_key.
If that's fast.. err, you're back to square one :/ or maybe you have too many indexes (which means the update process is spending all of it's time updating the fields & indexes).
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