Hi All,
I noticed a similar slowdown with 279. I was re-importing my 36000 items into a new library. after 10000 itesm each item took noticably longer to import, by the 20000th item it was taking sometimes a minute per item, sometimes 15-20 seconds. Now I put this down to the type of media it was importing. But eventually I got fed up, clicked cancel. closed MC then re-imported. It had remembered where it was up to and then completed the last 10000 or so items quickly (as quick as the first 10000 items).
Thankfully this isn't a show stopper as I don't tend to import my entire library often, it only gets slow on the import of the items, not when ignoring them, so import scans my entire library but onl finds a few extra files and adds them very quickly (90 or so seconds for 300GB and 36000 items)
HTH
Rich