Fast update of a lot of records in a database?

Your approach seems very inefficient - it looks like you're executing
thousands of update statements. Try something like this instead:
#---build a table 'updates' (id and value)
...
#---do all updates via a single left join
UPDATE bigtable a LEFT JOIN updates b
ON a.id = b.id
SET a.col1 = b.value;
You may need to adjust the syntax.