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
An error occurred while saving the commentDan commented
I've had the same experience. I'm using CC2018 (on Windows 10, if that makes a difference) and the issues have not been fixed. As convenient as Span- and Split-Columns are, they make editing the document nearly impossible. I sure hope they find a fix for this, but after so many years, it doesn't look like they can or care to fix it. :(