I pulled out a long ethernet cable, unplugged the Expresses, and started moving the Extreme around. In any room outside of the one I had it in the WiFi speed goes from 10Mbps to 30Mbps. Is this an acceptable WiFi speed drop from my ethernet 100Mbps?
If moving the radio increases throughput, then you may have strong interference in the original location. A lot of devices use 2.4MHz, cordless phones, microwaves, and other WiFi gear. If you live in a densely populated area with a lot of 2.4GHz networks, moving to another room might shield you from these other networks, thus giving you better throughput.
Your model A1143 supports 5GHz in addition to 2.4. the 5GHz range avoids interference and offers more channels (~150 vs 14 for 2.4GHz) to avoid conflicts with neighboring networks. If you have not already enabled it, turn on 5GHz. You can use the same SSID and password and devices should be smart enough to pick the best signal, but for test purposes, name it something different to distinguish it (MYWIFI vs MYWIFI-5G for example). For simplicity, use the same password on both MYWIFI and MYWIFI-5G. Then, test speeds on both the 2.4 and 5GHz bands.
5GHz is shorter range, and is less capable of penetrating walls and obstacles, but it is less prone to interference due to more channels and fewer neighbors with 5GHz routers, and the shorter range meaning neighbors signals won't reach your home. You can leave the channel selection Automatic and the Extreme or Express will use channels with less interference from neighbors.
802.11n is theoretically capable of up to 600Mbps speeds, far in excess of your 100Mbps ISP service. But, I notice you have the b\g compatible setting enabled, which is good if you have devices that can only use 802.11b or g, but can slow things down and is more susceptible to interference from older devices (even if they don't connect to your wifi). If all of your devices support 802.11n or better, change the 2.4GHz radio mode to N-Only. Same with 5GHz, set it to N-Only unless you need to connect 802.11a devices. Newer 802.11ac capable devices will be able to connect to N only networks without issue.
If you are unable to find the WiFi from devices after setting them to N-Only, they do not support N as a\b\g devices won't even see the N networks.
Your AP Expresses may also support 5GHz. the A1264 supported a/b/g/n in 2.4GHz only, the newer A1392 added 5GHz dual band. If the APExpresses can be connected by wired Ethernet, and run in Dual Band mode, then do the same as the above settings on the Extreme.
Also, the radio in your endpoints may be restricting your throughput. 802.11b devices will get up to 11Mbps, 802.11a\g up to 54Mbps, 802.11n up to 600Mbps and 802.11ac up to 1200Mbps or more. Newer versions are backwards compatible but older versions are not forward compatible. If your Expresses were "extending" the Wifi (see my previous post above), vs creating a roaming network (hard wired to the router), and if they are older expresses that only support b\g, then they would be effectively throttling your network down to a slower speed.
So, with the Expresses offline, and the N-Only and 5Ghz settings enabled on the Extreme, test speeds on the 2.4GHz network, and then connect to the 5GHz network and test the speeds. If the Extreme's signal can reach all parts of your home with these settings, don't bother with the Expresses. Else, either create a Roaming WiFi (Ethernet connecting the Expresses to the Extreme), and\or upgrade the expresses if they are the older model to get 5GHz bands in other parts of the home.
Exhausting, no?