It looks like your transfer rates are 270MBps for write (1024 / 3,78) and 602MBps for read (1024 / 1,71), and they are not possible with standard SATA2 and HDD..
My result are much lower (93/75Mbps):
Serwerek> time dd if=/dev/zero of=/volume1/DiskStation/bigfile bs=64k count=16384
16384+0 records in
16384+0 records out
real 0m 10.96s
user 0m 0.01s
sys 0m 1.45s
Serwerek> time dd if=/volume1/DiskStation/bigfile of=/dev/null
2097152+0 records in
2097152+0 records out
real 0m 12.62s
user 0m 0.52s
sys 0m 1.46s
can they be caused by the RAID 1 configuration? The disks are capable of transfer speed ~115MBps without any problem and I'm really confused about that situation.