I think, there is no option to disable parsing all project files on project load when using the UEStudio internal parser. Best is to ask IDM support per email, if there is already such an option or not and post the answer here.
If there is really no such option as I suppose, write a feature request email because for large projects with hundreds and thousands of files it is really better to have a "Parse all project files" command in the toolbar/menu which is executed when the user thinks it is necessary and not always automatically on project load, although the internal parser parses for symbols only in those files which were changed since last parse of the current file (according to help, not verified with for example Filemon).
When using CTags instead of the integrated IntelliTips parser there is already such an option in the project settings. So why should the IntelliTips parser not have also such an option.