You can select ‘Open Large Document...’ from the File menu to open documents that take too long to open in normal mode. Documents opened in large document mode are not parsed on open. Most visual editor features are disabled: the document appears in a non-proportional font in black and white, without syntax highlighting, folding, indentation guides, and so on.
If the computer runs out of memory while opening a document in normal mode, the user is given the option of opening the document in large document mode instead.
Files opened in large document mode will also be saved, unparsed, in large document mode.
This depends entirely on the memory and processing power of your computer. It is usually best to attempt opening a document in normal mode first.