What is the threat model if I run lots of untrusted JavaScript, but I only have a small amount of memory in other processes worth reading and I would notice sus

What is the threat model if I run lots of untrusted JavaScript, but I only have a small amount of memory in other processes worth reading and I would notice sustained high CPU usage? Is there an example in the wild of a spectre exploit stealing my gmail cookie and doing something with it? (Would be difficult since it's tied to other fingerprints like my IP) Or stealing credit card numbers when they're in memory after I place an online order?