Radically Different Wifi Speeds Depending on Apple Device

I'm using three Google Nest Wifi Pro routers and am getting 595Mbps at the first router connected to the modem. One room away, with a second Nest router, I'm getting ~350Mbps on my iPhone 14 Pro, but <20Mbps (?!) on my iMac Retina 5K 27-inch (2017) and iPad Pro 12.9" (3rd generation.) Everything is fully updated, including firmware, and I closed all programs and rebooted everything. Before I upgraded to the Nest system, I was getting consistent speeds. Is this due to the iMac and iPad being older and running slower wifi specs? Any help appreciated!