bad performance with ext4

Hello,
i create a new virtual machine.
Diskwrite with 1MB/s.
See screenshot with bad speed.

bad_performance.jpg

My image was before ntfs partition. So I reached up to 50MB / s when installing Windows.
In the version 2.3 but no vm starts with ntfs.
Poor performance with ext4.

Someone has an idea

System partition is a single disk
My image partition is ext4 with three hard drives in a software raid five.

Is the cause the software raid?

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