Intel 520 180GB sudden write speed degradation

[idata]

I have a brand new SSD Intel 520 180GB.

The sequential write speeds were around 220 MB/s at first (using SATA 2 here).

After just a few benchmarks and Windows 8 installation it's down to 170-180 MB/s.

The read speed is always the same, it's only the write speed that dropped.

I formatted and even completely erased the SSD with diskpart "clean all", but it didn't help. Ran the SSD Toolbox optimizer, scanned the drive.. no errors or anything, I just can't get the write speeds back to normal.

Any ideas?

I can post the pictures from benchmarks if you're interested.

Here's the screenshot from HDTune after the speed drop:

Does this look normal?

I'm using Windows 7 64, Gigabyte P55A-UD4, "native" SATA port (not Marvell).

[idata]

I installed the benchmark program you used. Read test went fine. When I tried to do a write test the program complained that write tests were disabled because I had active partitions on the disk and they had to be removed before write tests could be done. Removing/deleting partitions on my setup isn't an option because it's the only disc in my computer! I was just curious how my SSD 520 performance compared to your results.

[idata]

You can try with http://crystalmark.info/download/index-e.html# CrystalDiskMark CrystalDiskMark and http://alex-is.de/PHP/fusion/downloads.php?download_id=9 AS SSD Benchmark, which don't require deleting partitions.

Here are my results before and after:

Some say this is just normal degradation after writing incompressible data. In fact if I try with Atto, which tests with compressible data, it's the same as before.