Exporting 10 hour blank screen 4k video - 4 hours to export and estimated 600GB?

Hey all - I'm really into blank screen sleep videos and want to make my own. In FCPX I have about 30 seconds of intro follwed by 9:59:30 of blank screen and audio (either music or sound fx like rain and wind, etc). If I choose to export the file as "audio only", it's about 1.5gb. But if I include the video, the masterfile export is 600GB! If I switch to "web host", that estimate becomes about 90GB... if I downsize it to 1080p, that becomes 70GB.

I ran a test last night and exported the video as 1080P for Apple Devices... It took about 4 hours and the resulting file was only about 800MB.

Aside from the first 30 seconds of intro, there isn't even so much as a black screen title/slide...

Anyone know why it would take 4 hours to export such a simple video? Thankfully the filesize estimate is way off... Would love for any ideas on how to reduce the export time. Am I missing something obvious? Google hasn't been much help.

Thank you so much!
-Push