I solved it - after several days trying to figure it out. As often is the case, the problem wasn't anywhere I was looking, it was somewhere else entirely. With hindsight, it makes sense (mostly) - before it made no sense.
For a while I've been using Filezilla Server as an FTP server to accept the images downloaded by my security cams. Everything was working fine. Then I switched to a new router a few days ago. Things stopped working. I couldn't get any images, or could only get periodic images, or (as noted above) the Hikvision could only download one snapshot every 6 seconds instead of the set interval of one per second. Incongruously, my Panny cam kept working fine as it always has, however. So of course I figured the problem was with the other cam, a Dahua.
At the same time this happened the Dahua cam got a stuck IR filter. The Dahua could not download any images to the FTP server, although it could create directory names that remained empty of images. It's been really cold here (for us in northern Califronia), below freezing at night, and I figured the Dahua had died a complete death, losing the ability to send images to the FTP server and having the stuck IR filter.
So I switched out the Dahua for a newer Hikvision, and that of course solved the stuck IR filter problem, but only partly solved the problem with inability to download images: I couldn't get a faster rate than 1 frame every 6 seconds even though the cam was set to download an image every second.
Through all this the Panny cam kept working fine, with no change. Weird.
I blamed the router. I blamed the wiring. I blamed the old Dahua. I blamed the newer Hikvision. I blamed Filezilla Server. I blamed the cat. All of this was checked out, and many different software settings were tried on the cameras and the server and the router. No solution. The cat avoided me.
This evening, I decided to check the PC that's acting as the Filezilla Server. After lots of looking, I discovered that it thought it was on a public network, not a private network, so it was blocking all attempts to access it, even from inside my LAN. I changed the PC's network settings so that it was on a private network and enabled discovery and file sharing within my private network. Everything started working with the cams and Filezilla server.
So - now I see what happened. When I changed the router, the server PC saw it as a new network (which it was), and defaulted to choosing it as a public network and blocked all sharing from within the network. I didn't see this happen because the server PC does not even have a monitor attached - it's administered via Remote Connection from another PC on the network.
Why the Panny cam kept working I'm not sure - somehow it got through the blocking. I'm not sure why. That threw me off. And of course the Dahua IR filter failing at the same time also threw me off.
I post this all with some degree of embarrassment, but hoping it will be instructive for others who might encounter a similar issue. And I'm apologizing to the cat with fish dinner.
Les