According to Ben Supnik from Laminar Research “Framerate should be back to where it was in beta 3. Betas 4/5 were not deleting smoke particles, so over time the total number of particles in the world would grow indefinitely, until the particle system was using most of the CPU.”
“Flashing in the cockpit should be fixed. The environment maps for the new lighting use alpha in the interior render to indicate areas where exterior light comes through, e.g. the windows of the plane. Due to using the wrong variable, on every other recomputation of the environment map, the alpha channel would be left opaque, effectively covering the windows in black paper and darkening the cockpit.”
“Finally, perhaps most importantly, this beta features a rebuild of the XPLM, the DLL that loads plugins. Besides modernizing the XPLM to use the newest compilers, this rebuild fixes the interface with X-Plane for popup menus (needed to get menu check boxes and disabling to work) and for keyboard focus (e.g. so you don’t get two blinking insertion points at the same time when editing text in a plugin).”
Read Ben’s whole story at the X-Plane Developer blog.