Why does this require checking every item in every column? It looks like the layout algorithm greedily picks the column with the least total height so far, every time it adds an item.
columns: 1fr auto max-content
<!-- items -->
<div>1</div>
<div>somethingsuperlong</div>
<div style="span: 2;">something else</div>
Depending on the non-fixed column type (there are different rules for each type) you want to ensure their constraints are satified (typeically they are big enough to encompass the items).