This module allows to browse comfortably and to be able to display slow queries for the current page using Devel module at the same time .
The slow query log is not displayed on initial page load but is stored in cache for a short period instead, so it then can be fetched in a separate AJAX request. Note that query summary is displayed same as before so user can make a comfortable decision whether he does need to read the log.
Devel module gives 2 options of query log display to choose between:
the log is either displayed everywhere or not displayed at all. Having the log displayed everywhere is bad because of the size of added HTML. It makes page loading slow and also makes browser scrollbar useless because "content" becomes a very small part of a page.
The module was made as a development tool to analyze performance problems and catch slow queries early. Developers can use it while building a site.
For production systems it's much better to collect and analyze slow query logs at the DB level, because that gives much better, complete picture of site DB performance.
"devel_queries_ajax_querylog_lifetime" variable controls query log lifetime in seconds (its 10 minutes by default).