How should we evaluate our peers?
I'm only going to answer the first part of this question:


  How should we deal with this opposing constraints (evaluation is serious matter but time is limited)? 


It is true that it is often rather time consuming to do these evaluations. But I think ultimately, you need to apply the categorical imperative. In other words, you need to spend as much time and diligence on evaluations as you would expect your evaluators to spend on yours.

For a two-page application for a sabbatical, this may not be very much. But for someone applying for tenure, surely a thorough reading of at least the most important papers, along with some research about the impact of the candidate's work, is appropriate. This is thankless work, as you don't usually get a lot of credit for it. But it is nevertheless something that can't be taken lightly because of its outsize effect on the candidate and the importance of your evaluation. Consequently, if the case is not completely straightforward, if you find that you need to spend a couple of days on it, then that is what you need to do -- because you would your evaluators to do the same.