The offline map loading performance has been addressed in the latest ArcGIS Survey123 3.13 release. The only way to get the offline map to download in the field app is to use the linked content to link the map packages to the survey. Once the survey is loaded, the map packages are available to download from the 'Offline Maps' option in the field app. Other options to sideload the offline map packages are to use shared or existing map packages loaded on the mobile device, or publish the survey with media folder map packages, the basemaps are available in the basemap chooser.