Thank you for reporting the issue to us.
We have been able to improve the performance with span columns but it is till work in progress.
Please upgrade your InDesign application to the latest version to try out the incremental fix.
If you face any problems with the new update, do let us know in the comments.
You may also follow the below given link for the list of all fixed issues in Id 15.0
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If a solution was still not found, then remove these features!