Time Machine backups are extremely slow

Time Machine backups filled an older, smaller external HD, began deleting oldest files. TM backups began to be very slow, I assumed because of the deletion of older files. I replaced external HD with a new LaCie 3.0TB external HD (p9233). Backups are still very slow, take a very long time. It appears TM is backing up all files each time it runs - literally hundreds of thousands of files each backup when only a few files should have changed from one hour to the next. I have tried on and off as well as setting up TM anew, nothing seems to affect time and speed. Any help?