Your comments

Can you please send me the relevant crash reports?

Here's how to do that: http://feedback.textasticapp.com/topic/48133-how-do-i-send-crash-reports/
I plan to make this configurable in an update.
Currently it's not possible to deactivate the indentation of wrapped lines. You can disable line indentation however.

I thinks indendation of wrapped lines makes long lines much more readable, especially with code. It may be different if you use it mainly for Markdown though.
Try to open the same text file in another editor on iOS - even the built-in Notes app - and you will see that Textastic actually handles large files fairly well compared to other editors.


In order to support this, I would have to completely rewrite the editor to only map those parts of the file into memory that are currently displayed instead of loading and analyzing the whole file. I'm not going to do that, sorry. 

Sorry, a 5 MB is too large to syntax highlight on the iPad - especially on a first generation device. The iPad has no concept of virtual memory, so it's quickly running out of memory and crashing as a result.
Textastic hat keinen PHP-Interpreter und zeigt daher die PHP-Tags wie reguläres HTML an.

Für eine Vorschau von PHP-Dateien müssen Sie die Datei auf einen Server mit PHP-Unterstützung laden und z.B. in Safari die Remote-URL einsehen.