hard drive - What is the fastest way to resize a large partition?

Due to a new HDD Configuration I am currently handling larger backup/resize tasks with partitions between around 900GB, which are 70-90% full.

some background:

First thing I've noticed was, that the Acronis TrueImage version from Western Digital was extremely slow while running it under Windows 7, even though on high priority. To create a normal backup for 650GB of data (900GB partition), it would have taken 3 days! The same task done with the Boot CD version of this Acronis version took about 2 hours (SATA3 copy from one disk to another, both around 110MB/s).

Now, after I have done all my backups, I've wanted to remove some obsolete partitions and resize the leftovers to full HDD size. Of course, usually this takes quite some time - in this case for this 900GB partition, to extend it to 931GB (30GB+ from front, 1GB+ from end), it will take around 6 hours (using gparted)!

Had I known that earlier, I would have just restored the image. But no - first it showed a reasonable time of 1:45h and 0 of 1 operations, but after finishing 1:45h it started again, only this time with 4h to go, still 0 of 1 operations, but now it was copying instead of moving.

Question:

However, why has it to be this slow to resize a partition? I am asking for a good explanation. This has bugged me, since I started partitioning - why does it require to copy all the data around, can't it just stay in place?!