I had very slow wifi speeds today and I found that it was due to contention with multiple neighbours also using the same 2.5GHz wifi channel, channel 11.
I was under the impression that the Auto option should do a scan of the other wifi networks in range and choose an unused / lesser-used channel. However, it chose channel 11 which had become very contended, and rebooting the router did not fix the issue. Only manually setting the 2.5GHz wifi channel to channel 9 fixed the issue.
I had the same impression initially regarding how the auto channel selection algorithm works. Unfortunately from what I've read it's not a very smart algorithm and I don't think it even scans the bands. You are almost always better off selecting a channel manually after reviewing the congestion and available channels yourself.