Yes, that is what happens. I think that first it downloads all the stars, then it downloads the property for each.

Not all *Data functions are so slow. The new, Entity-based ones are much worse than the older ones.

It turns out that AstronomicalData is actually usable here

magn = AstronomicalData["Star", "AbsoluteMagnitude"];

This first downloads a big part (or all?) of the AstronomicalData dataset, which will be kept on the hard drive. The downloads completes in a couple of minutes. Once that is done, it's still not exactly fast, but it is unquestionably usable. It evaluates in about 22 seconds on my computer.

Compare that to StarData, which will re-download for every session and is plainly unusable.

Hi Szabolcs,

If I do:

magn = StarData[All, "AbsoluteMagnitude"];

I get something like this:

Which finishes in a minute or so (steps of 2500). But then it goes again downloading some more stuff but in steps of only 64 which takes indeed forever. Are you seeing the same? I agree, this should be within a second. But all the *Data functions in Mathematica have been notoriously slow since their introduction (V6?).

I agree, steps of 2500 (not to mention 64) seems ridiculously small for today's standards.

I honestly don't know what takes so long, is it the connection method, the protocol, the server, the databases?