I recently posted a screenshot showing the great speed of my AT&T Fiber connection. (And it is, indeed, that fast for wired connections.) But I noticed that on some days the automatic eero speed test was reporting quite different results.
Long story short, and after a very helpful exchange with eero support, an eero network running in bridge mode cannot produce a consistent speed test. This is because which eero module is used to perform the test is, as they say, indeterminate.
Some of my modules are connected using MOCA v1, which is capped at 100mb speeds. If the test selects one of these, obviously the result is going to suffer. In fact, this is what was happening on my network.
If eero is not in bridge mode, it will use the module connected to your modem/router. Perhaps someday I will be brave enough to try to use eero without bridge mode, but for now I believe I cannot, due to the funky way that AT&T allocates static IP support in their router.