You are right; may be nim-mode is exacerbating the slowness issue. I haven't come across this issue as I limit the number of chars per line to around 80.
Try using `nimpretty` that ships with Nim. With a `longlines.nim` ( [http://ix.io/1QnF](http://ix.io/1QnF) ), running `nimpretty longlines.nim` modifies the same file to this ( [http://ix.io/1QnG](http://ix.io/1QnG) ).
