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There are several reasons why updating via join in MySQL can result in slow performance:

1. Large tables: if the tables being joined are very large, it can take a long time to update the joined result set.

2. Lack of indexes: if the tables being joined do not have appropriate indexes on the fields used for joining, the query may need to read all data to perform the join.

3. Complex conditions: if the join condition has complex expressions or functions, it can take longer to process the query.

4. Locking: updating via join can sometimes lock the table and prevent other queries from accessing it, which can slow down performance.

5. Server load: if the server is already under heavy load, updating via join can further slow down performance.