I'm a trifle chagrined, though, as the reason my system had taken such a performance hit was that half my RAM was off-line. I'd been misremembering my specs - I bought the machine with 512K instead of 256K, so when they went to install the 1G stick in the slot that was reading as empty, they discovered - voila! - a second stick of 256K already occupying the slot, with a non-functional side-clamp. Hence the week-long stay in the shop, as they ordered and installed another new logic board to replace the faulty one with which they earlier replaced the one I fried in Idaho (after which said performance hit convinced me I needed to up my RAM).
I always have trouble with orders of magnitude. But I still feel kind of stupid for not figuring out what was going on. I blame PAWS.
Anyway, now I have a spare 256K stick of memory that I'm not sure what to do with.