The Dell uses 802.11n on the 2.4 GHz band. n supports theoretical speeds up to 400 Mbps, if I recall correctly. This is the reason I thought the Dell's 57.31 down may be unusual, especially considering the Kindle Fire performs better on the same band. Are there ways to improve a connection on the 2.4 GHz band? What are possible sources of interference?
Maybe I'll leave devices primarily for browsing on the 2.4 GHz band and move devices for media to the 5 GHz band. I'm not sure, though. The father often complained about dropped connections when we had only the 2.4 GHz band to work with.
Speeds of 802.11n on the 2.4 GHz will depend on if the router and the device connecting to it can use one or two channels and whether both the router and the device connecting to it can use one or more antennas. If the connection is only using one 20 MHz channel (typical...see more in a moment) and one antenna, then the maximum theoretical throughput is 72 Mbps. And since it is pretty impossible to achieve the maximum theoretical throughput on any network protocol due to a number of factors, a throughput of 57.31 Mbps on a 2.4 GHz 802.11n connection seems to be potentially consistent with a connection with just one 20 MHz channel and one antenna. In my experience, you can pretty much assume about an 20% loss of throughput from theoretical maximum assuming you have a great network connection (i.e. in the case of WiFi, relatively close with strong signal).
The way that 802.11n "speeds" up connections from that 72 Mbps level is to either 1) use a combined channel (i.e. combining two 20 MHz channels into one 40 MHz mode/"superchannel") or 2) using more than one antenna. For #1, this can be complicated as there are limited channels/segments in the 2.4 GHz range and it is a rather crowded frequency range (i.e. WiFi, Bluetooth, microwaves, potentially cordless phones, etc). As a result, it is very common for interference to prevent the ability to combine two 20 MHz channels into a 40 MHZ "superchannel" (so to speak). This means that you are more likely to be able to achieve more throughput with multiple antennas (aka SU-MIMO). But again, as I mentioned in my post in the your other thread, MIMO must be present on both the router and the device connecting to the router to be effective. And I believe in the case of 802.11n, not all MIMOs are implemented the same since the 802.11n standard has some wiggle room in the standard for how MIMO is implemented (at least that is my understanding), so even if both the router and the device support MIMO, there might be differences in how that MIMO is implemented which might mean that those two implementations of MIMO might not play nice with each other.