virtualbox 6.0.12 running super slow on debian 9 kernel 4.9.0-11-amd64

[anleapat]

Hi, guys:
Before, I run my win7 guest and centos7 guest on host debian 8 with virtualbox of version 5.2 and 6.0.0, it worked perfectly for years. My hardware: intel i5 4 core, 28G ram.
But few days ago when I upgraded my host to debian 9 kernel 4.9.0-11-amd64 and the virtualbox updated to version 6.0.12, it slow to the ground,
I running mysql in the centos vm, before I restarted mysql it take about 10 seconds, but now it take minutes, run one select script before it take 0.1 ms, now up to 30 seconds.
everything driving me nuts .
I have tried to decrease and increase the vms' cpus, ram, checked and disabled the 2D and 3D acceleration, actually I have never ever turned on it.
Now I give the win7 vm 6 processors 8G ram(before 8 processors 8G ram), and give centos7 4 processors 8G ram(before 4 processors 4G ram), but it make nothing change.
Anybody help me out.

[Martin]

Your CPU only has 4 cores available.
How did you manage to set more cores to the VMs?

You need to keep at least one core free for the host to handle a VM.
So any VM with more than 3 virtual CPUs will be very slow on your PC.

[anleapat]

there is total 8 in the panel, mine is i5 4590, I don't know if it's per core two sockets. Anyway, I have figured it out already, thanks you..

[socratis]

there is total 8 in the panel

That's wrong then. Actually you don't even have Hyperthreading:

00:00:01.689398 NumCPUs <integer> = 0x0000000000000006 (6)
00:00:02.416386 CPUM: Logical host processors: 4 present, 4 max, 4 online, online mask: 000000000000000f
00:00:02.416433 CPUM: Physical host cores: 4
00:00:02.416661 Full Name: "Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz"

And a visit to Intel's site confirms that. As Martin said, no more than 3, you're going to run into trouble, like slowdowns.

Anyway, I have figured it out already

Care to share the solution?

PS. For future reference you should ZIP your logs. It helps you, it helps us, it helps the server, it helps save a (virtual) tree or two...