Safari has major bugs in its XML system which can cause browser crashes when running the "Parse RNG", "Validate XML", "and "Report" methods. See the Known Issues file for more information.
The validator is not very fast, and it is even slower when it has to fetch external RNG files. Once the external files are cached the code will run faster.
Report generation is slow. You probably want to use "Only Report Errors" unless you have only a few hundred messages.
Using the "Verbose Errors" option can also slow the system down considerably, but it is useful if you cannot figure out why a document is invalid.