If you don't already have 17.0.3 and latest hotfixes, I recommend getting that.
In v17, we changed the GUI to use the Qt toolkit. In the original version (17.0.0), we would use the "raster" drawing under Qt, which did not perform well remotely. Try starting slickedit with the option -graphicssystem native, and see if the performance improves. We have made "native" the default since 17.0.1, however, you could still be getting something from your environment QT_GRAPHICSSYSTEM which may be causing it to select the "raster" drawing system instead of "native" X-Windows.