Poor performance with large .txt
Notepad++ was not designed for large text files, instead I recommend a program called glogg. found here: http://glogg.bonnefon.org/
and I swear to god if anyone dislikes this post and complains that I'm answering with a link then grow up because I did fully answer the question.
Try Ultra Edit Its the best in this case.
Features at a glance
Column / block editing
Multi-caret editing
Multi-select
Syntax highlighting
Integrated FTP client
Integrated SSH/telnet
Editor themes
Large file editing +4GB
File / data sorting
Powerful search
Supports regex
CSV data reformatting
Macros and scripts
File compare
Smart templates
I edit my huge files with Microsoft Visual Studio. It is super fast and handles any size! try it! You can download free version (Community edition)
UPDATE:
Notepad++ build 64bit is available now. You can use it.