Reading the paper it seems that this effectively messes with the cache to see how much it's being accessed generally, and based on that access pattern creates a

Reading the paper it seems that this effectively messes with the cache to see how much it's being accessed generally, and based on that access pattern creates a fingerprint that can be matched against the way another website uses the cache generally. Presumably, having several tabs open makes it pretty hard to distinguish one site through the noise. Unless it's Slack, then the fingerprint looks like a giant blob of constant data access...I jest I jest. From the paper: The main difference is that the Prime+Probe attack measures contentions in specific cache sets, whereas our attack measures contention over the whole cache. Specifically, our JavaScript attack allocates an LLCsized buffer and measures the time to access the entire buffer. The victim’s access to memory evicts the contents of our buffer from the cache, introducing delays for our access. Thus, the time to access our buffer is roughly proportional to the number of cache lines that the victim uses.