Have you checked the hardware compatibility list, and the hardware requirements? What are your PC specs? Qubes relies heavily on virtualization and your hardware must support certain features (and those features must be turned on in the BIOS) for good performance.
So confirm that you have turned on all the recommended hardware features, which should be on BEFORE install, you may need to reinstall if they weren't turned on as I'm not sure what Qubes does at install time if those features are off.
https://www.qubes-os.org/hcl/ https://doc.qubes-os.org/en/r4.3/user/hardware/system-requirements.html