It happens to me as well. It seems to have something to do with (a) time and (b) snapshots that you have created. It causes a machine to become really slow. Here is a "benchmark" to show what I mean. It writes zeroes to memory as fast as possible. On my VirtualBox virtual machine I get:
linux-s0l1:~ # dd if=/dev/zero of=/dev/shm/testfile bs=8M count=10
10+0 records in
10+0 records out
83886080 bytes (84 MB) copied, 2.0009 s, 41.9 MB/s
42 MB/s when writing to RAM? Wow that's bad. Let's look what the host machine makes:
tweedleburg:~ # dd if=/dev/zero of=/dev/shm/testfile bs=8M count=10
10+0 records in
10+0 records out
83886080 bytes (84 MB) copied, 0.0423951 s, 2.0 GB/s
about 50 times as much. Actually, even moving my mouse on the VM's display make the CPU go wild!
In VirtualBox, I deleted all snapshots and my virtual machine's performance went up from 42 MB/s to 150 MB/s. But I have other VirtualBox virtual machines on the same host with the same guest operating system that deliver 1.8 MB/s.
tweedleburg:~ # dd if=/dev/zero of=/dev/shm/testfile bs=8M count=10
10+0 records in
10+0 records out
83886080 bytes (84 MB) copied, 0.0423951 s, 2.0 GB/s
about 50 times as much. Actually, even moving my mouse on the VM's display make the CPU go wild!
In VirtualBox, I deleted all snapshots and my virtual machine's performance went up from 42 MB/s to 150 MB/s. But I have other VirtualBox virtual machines on the same host with the same guest operating system that deliver 1.8 MB/s.
