Diagnosis
You placed the new eero in a wireless dead zone. It has a weak connection to the main network, creating a bottleneck.
How to Fix It
Remove the new eero. The app's placement guidance is correct. Move it closer to another eero. You want it roughly halfway between the dead zone and a strong signal source, not in the dead zone itself. Run a placement check in the app after each move. Stop when the app says 'Great placement'.
If That Didn't Work
Your home layout may require a wired connection for that location. If you cannot run ethernet, consider using MoCA adapters if you have coaxial cable outlets or a powerline networking kit as a last resort. Connect the new eero's WAN port to that wired connection.
Heads Up
Adding too many eeros too close together can cause interference and slow down the whole network. Do not add nodes unless you have consistent weak signal areas. More is not always better.