Slow WiFi on iMac (Retina 5K, 27-inch, 2019)

I am getting download speeds of around 05Mbps on my iMac via WiFi. There are two other Windows computers further away from the router getting speeds of ~25Mbps.

Why is this happening?

Even before the lockdown, I was still getting consistently around the same internet speeds.

The router is located in the room below mine, and someone on Reddit said this is why, but I don't think so.

Time of day would make no change in speed before and after lockdown.