The shape-dbase reader does not use any SQL-driver to filter alphanumeric querys. An WFS-GetFeature request forces the full read of the shapefile. For big data sources the performance is very slow.
Although there is an alternative using other geodatabase (as postgis) providers, if the data source must be shapefile, It could be attractive develop the use of any DBF OBDC driver to filter the data similary as the spatial indexing does.
Mass transitioning all resolved issues that have not been updated in the last month to closed state