Setup:
120-150 jobs configured - all are part of a dependency tree
each job polls every 20 min svn
quietPeriod: 90 sec

Result: svn polling needs from 15 sec to 4 min to complete
It is unpredictable which job polls first and is inserted into the build queue. As some jobs needs a long time to ask the svn whether there are changes or not often jobs in the queue are startet (and fail!) before the last jobs with changes are inserted into it.

SVN polling needs to be done in build dependency order and not by chance.