MDADM slows down on Raid1 array

I am using Xubuntu 16.04 Desktop on a Core i5 machine with 16G RAM. Boot up is from a 120Gb SSD. I have installed an additional mdadm Raid1 array consisting of two identical WD 2TB disks, mounted as md0. The machine is (also) used as a general file, mail and web server inside a small home domain.

After booting up, read/write speeds to the Raid1 array are something like 50-70 Mb/sec, which is fine. When I store larger amounts of data to the array, write speeds start to drop. After writing about 3G of data, they are down to around 1 Mb/sec, and will remain like that until I reboot the machine.

I have made sure that there is no resyncing going on in the backgound. The array is reported to be faultless. Resyncing has been made faster by setting the speed_limit_min and speed_limit_max speeds to 50000 and 2000000 respectively. It takes about 4 hours to resync the array -which is fine.

CPU load on each of the 4 cores is less than 2% on average and memory usage is around 10% of the 16G installed.

It seems like a cache is filling up and not refreshing itself or something like that.

I would appreciate some advice on how to ensure proper read/write speeds of the Raid1 array on a continuous basis.

Thanks.

Karel