My apologies if you already know this but a harddrive works by spinning a platter underneath a head. The platter, being circular, is spinning faster at the outside than on the inside and therefore as it fills up it also slows down.
To me it sounds as if your harddrive has just become more full over time.
Ideally you would have a second hard drive to record to and I would suggest creating a small partition to record to and a second partition for storage of the recordings. The first partition you create would be on the outter edge of the drives platters and thus be the fastest. After recording you simply move it to the storage partition. This would ensure that you always have full drive speed for you recording session.
edit - sorry I missed that this was already a second. If you have the room, copy all the data off the drive, leaving it empty, and try a new recording and see if your speed has returned.