One possibility is outside interference with your wifi signal. Perhaps a neighbor has started up a new router, or changed the channel he's using to the same one you're using. If those two slow-linking sets are the nearest ones to your neighbor that could explain why they're the ones affected.
Try changing your router's channel assignment to a different channel to see if it makes any difference. You may need to experiment with different channels to see which gives the best result.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X
