You just write your parser to accept both line endings (that meet certain conditions) and semicolons as statement terminators. Going through and literally inserting semicolons shouldn't be necessary.
I also found this set of tests http://jsperf.com/asi-versus-msi
My results on Opera Next (v15) http://imgur.com/hkAopfX shows MSI fastest FWIW.
Python does make some exceptions, in that newlines within balanced paratheses and brackets are not treated as statement separators. But outside of that, this is what they mean.
As an example of the difference, consider the following JS:
if
(0)
statement()
if
0:
statement()
Another example. This is legal JS:
obj
.method()
In short, Python can always say "this newline is the end of a statement", except for some very easy-to-detect cases. JavaScript, on the other hand, parses until it encounters an error, then goes back to insert the semicolon and tries again. This implies some additional overhead. I don't imagine it's a significant amount at all, but there is at least a good reason to think that this might apply to JS but not a language like Python.