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I have found that if I Option-click the WiFi symbol in menu bar and the TxRate is low, clicking "Disconnect from <Netname>..." and then reconnecting to the network causes the mac to connect to the closest/best router.

Turning the WiFi off/on does NOT do it. Rebooting does NOT do it. Before El Cap, those methods would work. Now only Disconnect... works.

For whatever reason, the mac likes to stay connected to the most recent router, regardless of signal strength, as long as it can even barely "see" the router. Disconnect... tells it to forget the current router.
 
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