On the digitalIOPerformance GitHub page it says "With the release of Arduino 1.6, this library is obsolete and mostly a historical curiosity for now. It doesn't support the latest boards or the new core/library format, etc."

Does this mean development of digitalIOPerformance has simply stopped or does the 1.6.xx IDEs now incorporate the same methods used in digitalIOPerformance?

If the newer IDEs do not add the higher performance is there an alternative to digitalIOPerformance?

You can use direct port manipulation but that will make your sketch more platform specific. If you aren't sure you need higher performance you should use the standard digitalRead() and digitalWrite().