Since each pushState also creates a object reference that cannot be garbage collected as normal (every pushState includes a object to be passed to a popstate ev

Since each pushState also creates a object reference that cannot be garbage collected as normal (every pushState includes a object to be passed to a popstate event) I don't think it's too wild to suspect slowdowns to be caused by them using pushState instead of replaceState. It could of course be thousands of other things too (and in combination), but it's not a bad place to start.