The fake "processing" delay is likely there because averaged across all visitors (not just averaged across HN commentors and voters) it increases visitor confidence that a change has occurred in site tracking activities as a result of clicking that button and hence it's there because it net increases both customer confidence and flow through to the rest of the site, as annoying as it may be to HN readers.

The problem with having three sigma or more excess knowledge about a problem domain is that solutions designed for the center of the bell curve likely won't work well for the many-sigma outlier population, and the fraction of the population out in that many-sigma part of the curve is too small for providers to justify expending significant resources there. It's not uncommon for businesses to optimize for the center of the bell curve and leave many sigma outliers poorly served, as is happening here.