To fix a WiFi connection, it's important to make sure the Mac is within good range of the wireless access point, and, if necessary, to delete the WiFi connection to that access point from within the network settings of the Mac then reconnect the Mac to the same WiFi network.
WiFi Suddenly Slow? Best Ways To Fix Slow WiFi Speeds Sep 27, 2016 Troubleshoot slow Wi-Fi - Google Wifi Help Whenever a device connects to your Wi-Fi network, it competes with other devices for bandwidth (internet speed). A TV streaming Netflix, a laptop downloading, and an Xbox Live gaming session, all require bandwidth, and depending on your service, performing multiple high-bandwidth activities at the same time may result in a slower connection. If your Mac doesn't connect to the Internet over Wi-Fi Jan 16, 2020
How to Fix Slow Wi-Fi after MAC OS Sierra Upgrade
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How to speed up slow Internet downloads on a Mac
[SOLVED!] Very slow file transfer on Mac Shares (SMB/AFP Aug 29, 2018 Why is My WiFi So Slow: Tracking Down the Culprits - The Sep 19, 2019