I have an old system with simple older firefox -- It can collapse the given example without even noticing ... well not perceptible to myself ... I'd call it near instantaneous, also tried repeatedly as well as going though collapsing the rest but was unable to cause a falter. I do though have a deny list of sites that provide / contain scripts that snoop too hard as well as a couple I'm suspected set up as resident in the background where the behaviour was only corrected with quit and restart. Mind that deny list results in a few innocent sites not loading up, but I can live with that.