Performance issues with nested loops and string concatenations

Question

Can someone please explain why this code is taking so long to run (i.e. >24 hours): The number of rows is 5000, whilst the number of columns is 2000 (i.e. Approximately 10m loop

Answer 1

Supposing that textToWrite is a String, you should use StringBuilder instead. String is immutable and it is very ineffective to add small parts.

Ideally you would initialize StringBuilder with a reasonable size (see doc).