SlugHelper->isUniqueInSite() slow for installations with many sites and similar URL structures
If a large installation has a lot of sites / root pages, each with similar or identical page structures of a few levels, calling fixUniqueInSite() and especially fixUniqueInSiteForSubpages() in the DataHandler becomes slow due to the way potentially duplicate URLs are retrieved in SlugHelper->isUniqueInSite().
A fix could include a different or two alternative approaches for duplicate URL retrieval, taking into account the size and the overall page structure of the installation.
Also, the runtime cache could be used inside isUniqueInSite(), as this method is called recursively from inside fixUniqueInSiteForSubpages() and some of the necessary data (e.g. site definitions, root page association of a single page) should not be changed from the outside inbetween these calls.