Hello, I do not use DB2 Express, however, in Enterprise addition it is fairly easy to encrypt and password protect data, see the following link: Column-level encryption in IDS
For testing your performance I would do the following:
Create two tables, one with the video files unencrypted and the other unencrypted.
Insert the same data into each table. Then run queries against each table capturing the before and after timestamp of each query and compare the two. Not the fanciest method of checking performance but it works.