It sounds like you have a really slow web server. There may a small problem around the end of timed quizzes, but if there is, it should be a fraction of a second, not 18.
However, after taking a quick look at the code (Moodle 1.7), it seems OK for me. There should not be any problems even if your server is really slow. If you have the time and the necessary skills, it would be great if you could do a bit of debugging to try to diagnose what is going on.
Quiz
Timed quiz time problem
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