Sqlite3 Python: How to do an efficient bulk update?

I can do very efficient bulk inserts in Sqlite3 on Python (2.7) with this code:

cur.executemany("INSERT INTO " + tableName + " VALUES (?, ?, ?, ?);", data)

But I can't get updates to work efficiently. I thought it might be a problem of the database structure/indexing, but even on a test database with only one table of 100 rows, the update still takes about 2-3 seconds.

I've tried different code variations. The latest code I have is from this answer to a previous question about update and executemany, but it's just as slow for me as any other attempt I've made:

data = []
for s in sources:
    source_id = s['source_id']
    val = get_value(s['source_attr'])
    x=[val, source_id]
    data.append(x)
cur.executemany("UPDATE sources SET source_attr = ? WHERE source_id = ?", data)
con.commit()

How could I improve this code to do a big bulk update efficiently?