When using a customized Nintex Form within Office 365, the form might have an initial load time of 15 seconds or more.
As all custom forms are unique, here are some of the most common fixes for long load times with Office 365 Forms:
Reduce the number of controls. Make sure that all of the controls that are on the form are needed, and if there are any that were there for testing purposes, remove them.
Reduce the number of rules. Review the rules that are being used on the form, and if there are several rules that accomplish the same outcome, reduce the duplicate rules and apply the single rule to multiple controls on the form.
Break the form into multiple forms with redirects. If the form has logical break points, separate it into multiple forms, and use redirects to move the end user to the next stage of the form.