When I click a session in "TextView" that has a very big content size (1MB+), fiddler hangs for a long time.
I am not sure if that is because the data is loaded from disk, or is parsed.
In any case, I would suggest a partial, sequential load/parse to avoid the hang.
How long is a "long time"? What version of what operating system?
Fiddler keeps all data in memory, so unless the content has been swapped out, the disk isn't a factor.
What sort of content are you loading into text view (is it text, in English/European languages, or something else)? The Windows text editing controls have assorted performance bugs when rendering large quantities of text, especially when the text contains non-textual characters (e.g. binary control sequences, etc).