Regaining efficiency on older iPad I still have the very first retina iPad. Purely from empirical comparison, I have the impression that it has gradually become slower. At this point it is sometimes irritatingly slow, even for trivial -open-google's-maps-app- actions.

Are there any reports that this is connected to the recent old-iPhone-slowdown issues? Are there any reports of how I can regain the use of this iPad as simply a web browser and simple apps platform?
If I understand, you have a third generation iPad, which has an A5X processor. This is a fairly old CPU, as it was released in early 2012, but it should still provide a decent experience for web browsing.

Note that the A5X is 32-bit, meaning it cannot run a lot of new apps or iOS releases that are deigned for 64-bit devices. The latest version of iOS you can run is iOS 9.3.5, so I would first suggest you update to that version to get the most secure and stable experience possible on that device.

To address the iPhone throttling, iMore says that this does not affect iPads.
According to iMore, Apple only stated that a handful of iPhone models (devices from the iPhone 6 to iPhone 7 Plus) will be throttled at all. iMore also points out that the larger batteries in iPads makes this less of an issue.

If your iPad seems to be slow, I would suggest you connect it to a Mac or Windows PC and use iTunes to backup and restore the device. This will ensure a clean and up to date installation of iOS. While this may help, your iPad is a bit old, and it may just not perform well on recent versions of iOS. If you're willing to upgrade, the 9.7" iPad is a great option for its price and performance.