In this paper we propose Instance Filtering as preprocessing step for supervised classification-based learning systems for entity recognition. The goal of Instance Filtering is to reduce both the skewed class distribution and the data set size by eliminating negative instances, while preserving positive ones as much as possible. This process is performed on both the training and test set, with the effect of reducing the learning and classification time, while maintaining or improving the prediction accuracy. We performed a comparative study on a class of Instance Filtering techniques, called Stop Word Filters, that simply remove all the tokens belonging to a list of stop words. We evaluated our approach on three different entity recognition tasks (i.e. Named Entity, Bio-Entity and Temporal Expression Recognition) in English and Dutch, showing that both the skewness and the data set size are drastically reduced. Consequently, we reported an impressive reduction of the computation time required for training and classification, while maintaining (and sometimes improving) the prediction accuracy.