I am also feeling your griefs. I am using FCE 4.01 and trying to import AVCHD clips from a Sony HXR-NX5U. I have tried importing from Sony’s HXR-FMU128 memory unit and a sandisk SDHC card. It is behaving just as you describe. I have watched the activity monitor during an import and its interesting the watch the CPU load. It goes down as the clip progresses. To me, this is why the imports slow down to almost nothing.
I am using a 27″ iMac with the i7 processor and it is taking a 20 minute clip over an hour just to get 3/4 the way imported. Unacceptable. I recently tried footage from a Canon HF20 and got different results. I had it set on the same resolution and bitrate as the sony and a 20 minute clip from it only took 17 minutes to import.
It seems like the issue lies with the footage from the sony. I have tried contacting apple, sony, and b&h photo (where I bought the sony) and no one has been any help. They all gave me lame answers just to get me off the phone.
just thought I would throw in my findings.
Kristian