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As you can see, this option is NOT enabled by default as it may cause memory consumption problems for long running transformations. By default, every second a performance snapshot is taken for all the running steps. This is not a CPU intensive operation and it should not have any big impact on performance. However, if you have a lot of steps or if you take a lot of snapshots (several per second for example), there is obviously going to be some negative impact on performance that you should be aware off.
Also be aware that if you run in Spoon you will consume a fair amount of CPU power just by updating the JFreeChart graphics in the performance tab. Running headless (Kitchen, Pan, Carte, Pentaho platform, ...) does not have that drawback and should give you fairly accurate performance numbers without too much of a penalty (if any at all).
Saving step performance logging
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