Hi Olivier,
Thank you for contacting Syncfusion support.
We have analyzed the problem with your reported query and we are unable to reproduce more than 2 seconds delay to load SfNumericUpDown. It will take a second to apply the themes. We have prepared a sample, in this we have navigate to the page which contains SfNumericUpDown by click button. While navigating to new page, it takes less than a second to loaded it. We have attached the sample and you can download the sample from the following link.
Sample link: SfNumericUpdown_118015.zip
And also Kindly let us know are you using which version? if so it will help us to proceed further.
Please let us know if you need further assistance on this.
Regards,
Thirumurugan Arumugam.