mysql update too slow when joining multiple tables

Question

Hi, I have two tables. they looks like as follows: Id (int) Tags char(128).

the column Tags in table A does not have value. It is empty. The column Tags in table B has value. What I want to copy the Tags in table B to corresponding place of table A. the mapping is based on Id. My sql query is:

update A 
INNER JOIN B 
set A.Tags = B.Tags 
where A.Id = B.Id

There are about 2,000,000 rows in table A, and 50,000 rows in table B. The update seems very slow.... Could anyone tell me how to make it run faster?