Indeed, the upstairs guest bathroom in my house hates WiFi, as it shields itself from WiFi signals with several walls. Unfortunately, the Velop couldn't handle the walls as well as the second-generation Eero mesh system I tried.
I got an average of 48.2 Mbps downloads, or 24% of the baseline speeds from the main Velop in the living room. To compare, the second-generation Eero tested at 86 Mbps download speeds.
In this case, the solution is to buy another single Velop node and place it closer to the guest bathroom upstairs. That way, the extra node you bought would daisy-chain from the master bedroom node to deliver a stronger signal to the guest bathroom upstairs.
To be fair, the Velop in the guest bathroom upstairs would be fine for most internet usage, like web browsing, lurking on social media, or a single person comfortably streaming 4K resolution Netflix videos. As an example, I started watching a 4K YouTube video in the living room, then moved to the guest bathroom upstairs, and the video kept on playing without any hiccups when my MacBook Pro switched over to the Velop node in the master bedroom upstairs.
For average upload speeds, I got an average of 35.8 Mbps, or 98% of the base line living room Velop speeds, which is great.