Running Istanbul-instrumented JavaScript code is ~4 times slower in node 4.x (and 5.x) than on node 0.12.x.
Specifically, I have a Mocha test that takes ~500 ms to complete on non-instrumented code. The same test takes ~5000 ms on instrumented code in node 0.12.x. The same test takes ~20000 ms on instrumented code in node 5.x.
Any reason why the performance is so much worse in more advanced versions of node?
Note: In order to isolate the issue I ran the istanbul instrument command. It takes ~1 second to instrument. I then ran the Mocha test on the instrumented code without generating Istanbul reports or checking coverage.