Airport Express can't handle multiple devices I have an Airport Express connected to my modem (which provides 60 mbps download speed). The Airport Express is in DHCP + NAT mode, has one computer connected to it via Ethernet, and has other devices connected to it via Wi-Fi.
For some reason, when one device on the network is using a lot of download throughput to connect to the Internet (e.g., video streaming, downloading software updates, etc.), the other devices are unable to get more than minimal throughput to the Internet (e.g., simple webpages barely load). The device using a lot of throughput could be connected via Wi-Fi and even the computer connected via Ethernet will get little throughput.
Has anyone experienced this before and know how to ensure all devices have reasonable throughputs? I assume the issue is with the Airport Express. I have tried restarting the router along with reseting it and setting up the network again.
This is normal, especially with older routers like the AirPort.  TCP is a “greedy” protocol, it will take all that it can when it can, so if you have a node (computer on the network) consuming lots of bandwidth other nodes experienced high latency and low throughput.
You may or may not remember, but places with free WiFi would often have horrible connectivity when the “business” crowd showed up with their VPN clients.  Basically, they would hog the bandwidth whether they were transferring lots of data or not.  Many places blocked VPN traffic because if this.
You need a new, modern router
The way you fixed this was you configured QoS (quality of service) for individual clients and protocols.  More advanced routers would  allow “packet shaping” where the router would limit the amount of bandwidth per node and/or protocol.
Modern routers take care of a lot of this for you automatically. Many can allow you to prioritize  traffic.  For instance! http/https takes highest priority, but email takes lowest. More advanced routers (I use pfSense) allow much more granular control to limit bandwidth by node and by protocol/application.  A regular retail, consumer grade router should be enough for your needs.