In both tests exactly the same settings were used with 5 thumbnail items been added to the thumbnails view. As you can see v5.0.17.1 uses 26% of the CPU time while v5.5.2.0 uses only 11%.
You will see similar performance improvements when you use the VNC Manager's built-in VNC viewer.
We plan to release v5.5.2.0 build by the end of this week. Regards,