The channel width (20 MHz vs. 40 MHz vs. 80 MHz) directly impacts Wi-Fi throughput. If the AP is configured with a narrow channel width (e.g., 20 MHz), maximum data rates will be significantly lower than other APs using wider channels (e.g., 80 MHz). This matches the scenario where one AP achieves only ~75 Mbps, while others reach 500 Mbps.
B). DNS issues affect name resolution, not raw throughput.
C). Authentication failure would prevent connection, not reduce throughput.
D). DHCP issues would prevent obtaining an IP, not cause slower speeds.
References (CompTIA Network+ N10-009):
Domain: Network Troubleshooting - Wireless throughput issues, channel width configuration.