Thanks for reporting, I'll look into it. It sounds like it's not something that can be fixed on your end.
Unfortunately, due to Oculus discontinuing the Go it's not possible to publish updates of the software anymore since December 18th 2020. I'll see if I can make a version available for download which can then be installed by 'side loading', directly, or with the use of SideQuest. I'll let you know when I have a solution.
The next update of VR Tourviewer will include an optimization of the video player, to support videos with a higher resolution and higher bitrate. 8K video should then be possible, and your 5.7K video will also play smoother then.
If you want I can give you access to the Beta version of VR Tourviewer, just send me the email address of your Oculus account (in an email to firstname.lastname@example.org) so I can add you to the Beta channel of the app. The Beta version with this fix will be available in about 2 weeks.