Given that I was able to consistently see this issue when I was using Sublime Text 3, I'll see if I can come up with something more concrete to show/report this issue, over some weekend.
content = 'a'
with open('content.txt', 'w') as f:
for x in range(1, 18888789):
f.write(content)
The problem appears to be that ST3 slows down while loading this file which just has 1 huge line. If I tweak that script to change the content from 'a' to 'a\n' to include a new line within the content, the generated file loads relatively quickly and in reasonable time (around 6 seconds for the 37MB odd file).
I admit, this now I think can be considered a corner case but when I ran into this pretty frequently, I just assumed this happens with all files with such sizes. As for why I believed ST2 doesn't have this issue - I'm not sure anymore, because when I try these files now with ST2 it too behaves similarly.
I no longer work on that project which generated these files so had it no been for this thread, I would never have bothered to dig deeper into this and would have just stuck with ST2 :)