I've been thinking about your situation for a while, and I offer this as a possible explanation with a solution.
A Boost creates its own wireless network for your Sonos system, meaning that your other Sonos products communicate through the Boost instead of through your regular wifi. If you installed the Boost after your other Sonos products, it's possible that it created a network that's not being used.
In a perfect world, your Boost would be wired to your router/switch/whatever, and I recommend that you do so. This is a system that works. I use it at home with five Sonos products and at two work locations with quite a few more. BUT, I had to factory reset all of my devices and re-add them when I introduced a Boost into the mix after the fact.
If this is the case (that your other Sonos products are not communicating through your Boost), you need to (basically) rebuild your Sonos network. Unplug everything. Wire the Boost to your router and make sure your Sonos app recognizes it. This is a critical first step.
Once that's done, plug in a component and execute a factory reset. You can find the instructions for each device by searching the Sonos site. After you reset it, adopt it with the Sonos app. Go thought this process with each device. When you're done, all of your devices should communicate through the Boost, significantly reducing the traffic on your regular wireless network.
I absolutely cannot guarantee that this will work, but your description makes it seem like either:
- Your wireless (and maybe wired) network stinks, or
- Your Boost isn't doing what it should
Good luck!
Tim