Performance of CSV writer

[Arlofin]

The CSV export of data frames (via method SaveCsv) is very slow.
As an example out of my practice (based on v3.0.0): A 50,000 x 100 data frame with a resulting CSV file of 20mb size took 1min 20 secs to produce on my system. Column Types are 2/3 numbers (some integer, some double precision float) and 1/3 a two-valued discriminated union.

[Arlofin]

I figured it out: The default implementation of ToString() for DUs is very slow. After overriding it with a custom implementation, the previously mentioned data frame serializes in 6 seconds. This is still not impressive (around 3mb/sec), but at least makes it usable.