Thank you for getting to us with this!
The initial loading time of RadRichTextBox is influenced by its usage of MEF
which is used to load extensible components like the format providers and UI widgets. This can be slow in some situations and you can improve the startup time by changing the default catalog used by MEF
as described in this help article
We are currently planning performance improvements on some areas of the control and we will do our best to release them as soon as possible.
Also, please take a look at this help article
about performance in RadRichTextBox.
I hope this helps! Let me know if you have other comments or questions.
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