This is perfectly normal and the expected behaviour.
After crossing a region border (or after a far TP, or after login), the viewer auto-refreshes the objects visibility to fight "missing" (or disappearing) objects and objects near the border that get moved out of place (and therefore appear in a wrong place on the screen). Viewers not doing it simply suffer from these well known "glitches" (likely due to a race condition in the objects list refresh and the sending of the "interest list" data by the sim).
This incurs a short freeze (normally, well below one second: around 250ms or so, at 256m draw distance), but the more objects in the object list and the slower your CPU, the longer it takes...
You may control the delay after the login/TP/sim border crossing via the "ObjectsVisibilityAutoRefreshDelay" (setting it to 0 disables the auto-refresh), which defaults to 2 seconds (if objects are still missing after the auto-refresh, you may want to try and increase that delay to 3 or 4s).
Note: I split this into a new FAQ thread, since it had nothing to do with the experimental viewer.