Known issues in the Plasma 5.2 release.
Neither of these will be shipped in the stable tarballs.
Performance of Plasma 5 is heavily dependent on specific hardware and software configurations and usage patterns. While it has great potential, it takes time to wrangle this out of it and the underlying stack is not entirely ready for this either. In some scenarios, Plasma 5 will display the buttery smooth performance it is capable of - while at other times, it will be hampered by various shortcomings. These can and will be addressed, however, much is dependent on components like Qt, Mesa and hardware drivers lower in the stack. Future releases are expected to bring significant performance improvements thanks to fixes at all levels.