You're right. Playing game's off USB is slow because the USB bus on PS2 is version 1. You can get faster loads playing games over ethernet or through the memory card. This video does a good job explaining the issue and solution.
Play PS2 games from a Memory Card? | MC2SIO | MX4SIO
There's listings on Amazon for a "MX4SIO SIO2SD SD Card Adapter" for $11 with positive reviews from people playing backups. I've never used any memory cards loaders myself so I'm not sure how they differ from the MC2SIO featured in the video.
Your purchase of a FMCB wasn't redundant. The FMCB goes in slot 1 and SD MC in slot 2.
The video stresses that FMCB must be on an official Memory Card. If your FMCB is an unofficial card you can run the latest FMCB installer from your card and install another copy of FMCB to an official card.