Large "albums" (1000+ tracks) load quickly and correctly without lastfm.ng plugin enabled.
Enabling lastfm.ng results in a host of error messages in the log, and eventually Picard's actual memory use will swell to dozens of gigabytes, even starting to use caching on the hard drive.
Once Picard starts to use disk-caching, the application is unusable.
At a guess, it looks like some sort of lookup object is being created, and then not released, in the case of an error. I can't read the error logs well enough to be sure, but the amount of memory being used, and the rate of growth over hours, suggests something is leaking or being duplicated or expanded. Presumably, the memory is freed when smaller albums complete, but that's not happening here.