I have had this come up twice now in rather significant ways: someone is being considered for a position on a relatively powerful committee and the various candidates are of course very busy. For some reason, the decision was almost made (luckily some intervention occurred!) that a particular woman was just too busy and would turn us down and should not be asked. I have had this come up in a context where a senior male was making the nominations and where a mid-career female was making the nominations. The mid-career female mentioned that she probably shouldn’t be “protecting” the junior woman in this way. I found this to be interesting: are we not trusting the person in question, whom we regard as highly qualified for the committee, to make the right decision about serving?
I tried to do a quick internet search on the topic and came up with interesting articles about how women were too busy for various other things but didn’t find information on this.
In both cases, however, I suggested to the nominator that the candidate should be able to make the decision herself if she is too busy.
It might be that this particular opportunity is very important to her and she would step down from something else to do it (who knows!).
Often just being invited to serve can give someone a boost to their career, so we have to be careful not to withhold those invitations. We might inadvertently "protect" them from career advancement.