What happens whne you Ctrl+End is that SyntaxEditor needs to run the lexer on the text all the way up to the point at which you are displaying. That is because in many languages you could have something like a /* up at the top that could affect everything down at the end.
When I run a test using our normal free dynamic C# lexer, it takes a while but it's all trying to lex the millions of tokens.
When you run this file with the .NET Languages Add-on, it takes a while too but it's also running a parser on all these tokens in addition to all the normal lexing for syntax highlighting.
If I change it to Plain Text language, it will show instantly at the bottom because in that mode, it skips all lexing and of course the related syntax highlighting.
Unfortunately I'm not sure we can do anything else to speed up enormous files like that since the lexing does need to occur. The only idea would be to have the wait for the lexer to complete to bail out after some point and just show the text unhighlighted at the bottom. Then continue it on somehow in the background and update it when appropriate. That would require some major refactoring of how our code works right now though.