@Ryan Thank you for your reply.

I'm connected to the 5GHz band.

Router is sitting on a wooden table. Underside I would describe as hot, but not uncomfortable to touch or hold.

Behavior only began in the past month.

No recent firmware upgrades.

I *may* have messed with some settings before symptoms manifested, not sure.

A new duplex neighbor has moved in roughly during the past month or two. 

I'm comfortable with command-line tools. Would be curious to know how to diagnose any interference.

  @Ryan Actually, now I'm wondering if this may be specific to my MacBook Air:

I left the ping running all night on my MBA. In the morning I spent a half hour using my iPad (WiFi) to access the Web, with consistently good response. Then I opened the MBA and looked at the ping output, and I saw that the previous 8 hours had flawless 30ms response times with no timeouts. Within a minute or two of starting to use the MBA to browse, I noticed a slowdown and saw that the ping responses had become terrible. It was clearly triggered by my beginning to use the laptop.

  @claitch 

If it is triggered by the laptop, is there anything that would create a lot of traffic such as Cloud backup or a backup to the router's USP attached drive?

  @ArcherC8 No, I do my backups manually.

I'm currently trying to find a network sniffer to run on the iPad.

FWIW, I have a ping tool on the iPad. Ping response is fine when the laptop ping is okay, and it suffers when the laptop ping is suffering. So using WiFi on the iPad doesn't seem to cause slowdown, but it seems to be affected by it when the laptop is slow. 

  @claitch 

You could move devices one at a time to the 2,4 GHz and test for a bit to see if the issue moves or you could power off devices when the ping is high to see if that makes a difference.

  @ArcherC8 Thanks -- I have switched the laptop from 5G to 2.4G. Will continue to monitor for the next 24 hours.