Your post doesn't say, but did you try running dosbox again without using DBGL at all?
Removing you .conf file and starting dosbox clean from cmd-line?

Yes, I uninstalled everything (making sure all dirs were gone) and reinstalled 0.70 only, from the archive I had success with the first time.

Try different output= settings in dosbox.conf. Check the processor usage
in the task manager when running some heavy game (duke3d or something).
Does it work better if sound is disabled?

Tried them all, makes no difference.

But! Mystery is now solved! The cause of the problem was the USB joystick and joypad, which I forgot I had plugged in after the first attempt on 0.70. Removing them made everything go smooth again.

I have no problem with joypad/joystick in my SNES/PSX/Amiga emulators, or in modern PC games, so I guess it must be a DOSBox issue.

That's indeed odd. Could you see if it's caused by one of them? Both USB, right?

They are both USB, yes. I have two USB slots available, and when using the Tesun joypad in one particular slot, I get problems. Change to the other and all is fine. The Competition Pro joystick can be in any slot without causing trouble, and I can use both at the same time as long as the Tesun avoids that certain one.

My Thrustmaster Firestorm Analog 3 Gamepad (USB) is causing the exact same problems. If it's plugged in, the performance is decreasing dramatically.

The 20070225-Release (0.66RC3) by ykhwong seemed to worked without any speed issues.

If comparing older cvs builds please use clean builds.

Does it happen with fixed cycles as well? With all cores?

Changing the core and/or fixing the cycles doesn't help. It's still slow as hell, if the gamepad is connected - even at the dos prompt !!

I simply didn't test other clean builds, because of ykhwong's cvs was the only post-0.65 build I've used so far. And short after I downloaded it, 0.70 was released.

system specs:
Clawhammer 3000+
Gigabyte K8NS (NForce3 250)
GeForce 6800
Audigy
WinXP SP2

edit:
I tested 3 different builds(01.02, 25.02. and 08.03.) available at http://builds.tharos-online.de, but all of them offer bad performance.

My 0.70-DOSBOX is running at normal speed again. 😎

I finally was able to find out what was causing my slowdown problems. Some weeks ago I connected my old Suncom F-15 flightstick via gameport and had to add it manually to the gamecontroller list. After unplugging it, the joystick stayed on the controller-list with the comment "not connected". But when DOSBOX starts it reports this joystick (ykhwong's release doesn't !). So I deleted the joystick from the list and now the problem is gone! The only confusing thing about it is, that the slowdown-problem only appeared, when a "second" gamecontroller was connected - the USB-gamepad...

So in the console window are two joysticks displayed when that pad (only) is attached?

Exactly. The old Flighstick is not attached to the gameport, but still in the gamecontroller-list in windows, and both controllers are shown in the console window in dosbox. But if the gamepad isn't connected (the flightstick is still in the controller-list marked "inactive") dosbox doesn't recognize the flightstick and runs with normal speed.