Hi Sudha,
Thank you for your interest in Syncfusion products.
I am afraid that I am not able to reproduce the issue of 'CPU is 100% when UPdateUI is called with large number of menu items'.
Please refer the following steps I have followed to reproduce it.
1) Created number of menu items in XPToolbar.
2) Created number of CommandBar objects.
3) Set xpToolBar1.UpdateUIMFCStyle to true.
Here is a sample for your reference.
http://websamples.syncfusion.com/samples/Tools.Windows/F69428/main.htmPlease refer the above sample and let me know if I missed out anything to reproduce the issue you specified.
Regards,
Jaya