I have asked our QA engineers to check again the behaviour with the missing right border. According to them - everything works fine with the new version. Maybe we are missing something. Can you share some details e.g. steps to reproduce as we are no more able to expose this bug after the latest fixes.
Regarding the performance: Indeed with large amounts of columns we have a known issue of degraded performance with the merged column headers. This section of RadGridView does not implement virtualization( yet) . We are working on this and are going to include some improvements on this matter for the service pack coming in less than a month.
the Telerik team
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