"Is it possible to have a Windows VM without any lag".
Yes, I can confirm that it is possible. I run Windows 7 completely lag free on Qubes OS, it feels very snappy and instant, just as if it was running bare metal. Both with or without seamless mode.
I've installed Qubes Windows Manager completely according to the official Qubes guidelines.
My setup that runs Win7 smoothly
- QVM instance: Windows 7 64-bit
- CPU: i5 6500 3.2GHz (default set to two cores).
- Total system RAM: 24GB
- QVM Win7 memory allocation: 5GB.
- Disc allocation: 100GB (Make sure Windows has plenty free space).
- Graphics: Intel HD Graphics 530 (CPU integrated graphics)
- Triple monitor setup, all controlled by Intel integrated graphics (HDMI/HDMI/VGA).
- Seamless mode, sometimes on, sometimes off.
In case you use VGA monitors, then VGA monitor in my case causes some issues that impacts screen tearing a bit if used together with HDMI, only a slight second when moving app between screens. It is slightly annoying though, and I haven't looked into that yet. Though just be aware that VGA monitors might not play too well with HDMI/Display monitors, in case you use a setup like that.
You don't need 24GB memory, i.e. I got more than plenty, you will be fine with less. How much is a good guess though, I would say total system 8k GB memory is fine (Qubes 4k, Windows 4k each is fine for normal use), just don't open too many memory hungry VM's at the same time.
My CPU and its integrated graphics isn't top notch, but not all bad either, so if your system is anywhere near these specs, it is probably a software/driver/Bios-setting issue and not hardware performance related.
I found in my early Qubes days that Nvidia with the nouveau driver was laggy as hell, to put it mildly. My nvidia GPU that I first ran Qubes on was GTX1060, which is now retired from my Qubes setup. The GTX 1060 with nouveau ran though, barely. It was by no means smooth, neither in Dom0 or any QVM's, one big hell of lag. Some older graphic cards than 1060 might do better with the nouveau drivers, but the newer the card the less perfect it will likely run in Qubes (i.e. might be the case in your situation). The moment I took out my monitors from my nvidia card and instead plugged them to my motherboard internal graphics, I became completely lag free, and everything was really smooth (Shutdown Qubes first, so it can boot proper graphic drivers). I get stressed by lag, so I can definitely say that the contrast was big to when I used nvidia, I feel nothing now with Intel graphics, it's smooth. As many has said before with Qubes, Intel or many of the AMD graphics, just works. Nvidia with their monopolistic product designs is a big pain in the *** as they try to shove their market fragmented profit maximized proprietary garbage down our throats... *ahem*... we lack competition (Go AMD!).
Anyway, In Bios, in case you have such a setting, then make sure to check if on-board graphics are set to CPU, and not GPU. Furthermore be vary of any "shared" memory features between CPU/GPU in the BIOS, when I turned mine on it made Qubes's graphics teary and really laggy. This might hypothetically be more or less laggy depending on the hardware used, make sure to test the difference by turning it off if you got it on.
So if you got multiple graphic cards, this may be a source to your problems, especially if it is nvidia. If you are dual-booting (despite that it isn't the best to do in terms of security), then you can still make use of your nvidia card in other OS's, i.e. Windows should still detect your nvidia card. If you are running it on a desktop instead of a laptop, then you can use a KVM switch to switch between CPU/GPU graphic cards as you dual boot.
Qubes OS doesn't need heavy graphics anyhow, not as long as heavy graphics industry users, or gamers, isn't a supported audience target group by the Qubes developers.
Off-topic: However it has me puzzled why nothing more hasn't been done to support heavy graphics, it would make many more potential users interested in Qubes OS if it was possible to run heavy graphics on Qubes, such as games. Perhaps the security issue it creates is just too big and unfix-able or time consuming *shrugh*
Lagging (pun intended ;) a bit info on your hardware setup, so I took the liberty to make guesses. I hope any of this was of help to you, I had a laggy Qubes too before I found the proper setup (i.e. ditching my nvidia card).
P.s. if you haven't tried already, try experiment and reduce the resolution to something less, see if it makes any difference.