compress to another disk, then move back slower than operating on single disk

Question

Could you help me understand performance differences, I've experienced compressing some data?
The story is that I compress various files (20kB-600MB) in 2e20 chunks from one LUN to another.

Then I move back compressed file to primary LUN.
Whole operation is slower (~100ms for X0+ MB files) than working on single LUN.
I expected disk seeks to kill single LUN performance, I was wrong...
machine : RedHat with ext3, ~500MB/s IO

Thanks
P.

EDIT:
My benchmarks covers:
time = read time + processing + write time + move operation
the only difference is:

1. read time (LUN_1) + processing + write time (LUN_2) + move (LUN_2 -> LUN_1)
2. read time (LUN_1) + processing + write time (LUN_1) + move (LUN_1 -> LUN_1)

and approach nr. 2 is faster
benchmark covers (7k files)

I know that move on the same LUN is just a inode write,
but I expected read(LUN_1)...write(LUN_1) seek overhead be greater than move(LUN_2 -> LUN_1).