Hi,
recently (can't exactly pinpoint when), working with smart objects (whether in PSD or PSB files, usually in 16-bit ProPhoto RGB) got much, much slower. All types of compression are disabled, as reading stuff from my MacBook Pro's SSD is much faster than compressing/decompressing.
What's curious is the fact, that when saving the smart object as seperate document, both opening and saving of that file are much, much faster than opening the embedded smart object layer. Could this be because of, for some reason, compression is suddenly forced onto embedded smart objects?