If your environment hosts large (e.g., media) files on USB storage devices, you should ensure that your configuration allows sufficient RAM for read-ahead processing of large files, such as MP3 files.
You can change the configuration by adjusting the cache and vnode values that devb-umass passes to io-blk.so with the blk option.
A reasonable starting configuration for the blk option is: cache=512k,vnode=256. You should, however, establish benchmarks for key activities in your environment, and then adjust these values for optimal performance.