Hi Stephane,
I am afraid that I am not able to see the issue of controls being slowly loaded in the Form when TabbedMDIManager is used.
Please refer to the attached sample that I used to test this issue using v3.3.0.0:
http://websamples.syncfusion.com/samples/Tools.Windows/F69471/main.htmCould you please modify the attached sample so that the issue could be seen clearly? This will help us to resolve the issue soon.
We appreciate your interest in Syncfusion products.
Best regards,
Golda