I've noticed that when a (OGR) layer has a filter expression applied on it, processing algorithms perform worse. I have tested several polygon layers with some thousands of features. If I do, e.g., an intersection, it takes a lot more if I apply a filter that reduces the number of features to some dozens, then without filtering.
I guess it can be caused by the extra work that the feature irerator has to do with the filter (request with expression) but it's an unexpected behaviour, at least for many of my courses partecipants. Tested on QGIS 2.14.10 and 2.18.4. giovanni
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