After all, the small antenna of the dongle is now _inside_ the laptop, and very close to high-bandwidth USB data-lines.
(And possibly other sources of interference, depending on the expansion port it's connected to. If I use a right-hand mouse and the card is in a left slot, the signal would more or less have to travel through the entire laptop...)
Probably not significantly different from the dongle being plugged into one of those dual-port sockets. That being said, having a BT dongle right next to an active USB 3 cable is known to cause interference, see [0].
I'd be curious about this as well, I have one of these Logitech dongles and if I plug a USB3 device in the port next to it (say a SD card reader or external drive) the logitech device, my mouse in this case, becomes basically unusable due to the interference.
What's between the extension card and the framework laptop body? The dongle holder is 3d printed, so then RF Signal should escape relatively well compared to metal, no?
I don't know where it would escape to without interference.
In the direction of the outer module edge is a USB-port (with a large metal grounding) which possibly transfers high-frequency USB-data, in the other directions is the metal body and the laptop PCB (with its own RF-interferences bouncing inside of the chassis).
WiFi, in comparison: I don't know the exact design of the framework laptop, but on others the (two) Wi-Fi Antennas are usually placed on top of the display, to be as far away from the PCB (and the users' hands) as possible.
After all, the small antenna of the dongle is now _inside_ the laptop, and very close to high-bandwidth USB data-lines.
(And possibly other sources of interference, depending on the expansion port it's connected to. If I use a right-hand mouse and the card is in a left slot, the signal would more or less have to travel through the entire laptop...)