Hi,
I've been using a VirtualBox's Windows guest on Debian testing desktop for a while. Now I'm trying kvm. It seems to run faster when connected from localhost. Though the performance over a WLAN connection is just awful. The screen refresh rate is so low so that the VM is barely usable. Even for the desktop screen. The connection is done via spice (Remote Viewer client) for both localhost and WLAN PCs. Is that a limitation of spice protocol/spice client itself or it can be tuned? I can connect to the VirtualBox's guest from the same WLAN PC over rdp without any problem.

p.s. I can setup a bridge and connect to the kvm guest over rdp, but using spice is a more straightforward way. Besides using another remote desktop protocol than the one provided by kvm itself sounds like a kludge to me.

How does spice work locally for you? If you haven't configured the guest for spice, you'll see bad performance locally as well, and that would indicate you need to set up the QXL video adapter and the QXL drivers inside the guest. Without them, spice does not work very well, just like a real machine will have crappy video without the driver configured.

The local performance is Ok. I don't feel any difference to a native install. The guest is set up to use the QXL video adapter, VirtIO hard drive and ethernet card, both QXL and VirtIO drivers are installed.

Just tried to use a 1GBit LAN connection in place of the WLAN on my laptop and the lags are gone.

well then this is definitely a problem of your wifi connection, not spice

After some searching and experimenting it looks like Spice's designed either for a localhost connection or for a LAN connection. Trying to use a VNC server in place of Spice has solved the issue for the WLAN machine. Hope some sort of a slow-connection spice version will be developed some day since from what I've read about spice it sounds to be far superior to both VNC and RDP.

It is superior to both, and it works great even over WAN. WLAN is a very special case