This setting controls the number of threads/tasks that are reserved for the processing of fast or critical tasks only.
This feature is intended to better handle situations where NCrunch must perform several long running tasks concurrently, which can potentially block up the processing queue and make the engine unable to switch tasks to pick up higher priority work that may have appeared after the long running tasks were begun.
This setting reserves a specific number of threads/tasks that will never be used for executing long running tests unless they are specifically marked as high priority in the NCrunch UI. The reserved threads/tasks allow NCrunch to respond quickly to fresh changes made to the solution that may require high priority work to be done.
Where possible, always try to have at least one fast lane thread. This will make the engine much more responsive while you work.
If you are trying to maximise the number of long running tests executed by NCrunch in parallel at the expense of making the engine less responsive (for example, if you have a large test pipeline and you are going to lunch for an hour), you can set this to 0.
This setting is useless unless parallel execution is enabled.