Well, if you need an easy repro, there is this page https://www.kmart.com.au/search/?searchTerm=shirt
Every time it is refreshed it increases Pale Moon's memory use by about 100mb. This memory never gets released (even if the tab is closed), and the browser very quickly starts exhibiting symptoms described in the original post (high cpu and memory use, sluggish). There is eventually a ghost window for every refresh of that page.
By the way, Firefox is able to garbage collect it (without closing the tab and killing the process), so maybe there is a glitch in Pale Moon's GC after all.