node.js uses V8, although I guess it’s not the same as the current version in Chrome, but I guess it’s a good indicator of node’s performances on the subject.
Secondarily, you’re using forEach, which is quite handy when developing but adds a callback for every iteration, and that’s a (relatively) lenghty task. So, if you’re interested in performances, why don’t you just use a normal for loop?
for (var i = 0, keys = Object.keys(object); i < keys.length; i++) {
// ...
}
This yields the best performances you can get, solving your speed problems in Safari too.
In short: it’s not the conditional, it’s the call to hasOwnProperty that makes a difference. You’re doing a function call at every iteration, so that’s why for...in becomes slower.