I'm evaluating new Jenkins Team Concert Plugin as it has better RTC integration. But I found the performance is terrible. As an example, it took 42m37s to fetching files in one of build (see screen shot below) even there is no changes. The old Jenkins plugin takes less than 1 min in updating sandbox! 

When JBE updates the build workspace, it does so using a multi-component replace, not a regular accept, so discarded changes are properly removed, and there can be no conflicts. It also creates a snapshot after this to capture the state that will be built (except for personal builds). You can think of JBE as another developer that just brings his workspace up to date with what's in the stream (using replace, not accept). This provides the necessary isolation of the build from ongoing changes in the stream (the replace is done atomically).