@cfastner 

Hi,
When you notice a drop in speed from the main Deco, please disconnect the main Deco from the front modem, connect a PC instead, and check the download speed from the front modem. If the speed is around 90 Mbps, check whether rebooting the front modem helps.

After the download speed drops on the wired computer, what's the download speed on the wireless clients connected to the main Deco?

If both wired and wireless download speeds dropped from the main Deco, it is possible that the associated speed between the main Deco and the modem has dropped from 1 Gbps to 100 Mbps, resulting in a download speed of only 90 Mbps from the main Deco. The associated speed is related to the physical connection; try using a high-quality Ethernet cable between the main Deco and the front modem, ensure they aren't too far apart, and switch the ports used on both the main Deco and the modem to see if these help.
How to check the associated/link speed on a wired client

Best Regards