Pivot rendering in the Koltin notebook plugin is slow

While working with a relatively big dataset (~10m rows), I observed that rendering the table for a pivot result is extremely slow—about 10 seconds for a 10m rows dataframe.
After a brief investigation, I found out that the problem is with converting the pivot object to a dataframe. That is implemented like that `pivot.frames().toDataFrame()`
TODO: investigate what is the bottleneck and optimize the Pivot->Dataframe conversion process

Start at the Kotlin notebook plugin path that renders pivot results and inspect the `pivot.frames().toDataFrame()` conversion. Profile the conversion with a dataset near 10m rows to identify the bottleneck; done means pivot-to-dataframe rendering is measurably faster, though the issue names no specific files or tests.

Written by the indexing model from the issue text.