Incorrect file associations could be the result of underlying issues within your PC system. As such it is highly recommended to scan your PC for invalid registry entries, unused processes and other unfavourable system settings to identify slowdown issues.
The QCOW2 file type is primarily associated with 'QEMU' by Fabrice Bellard.
QEMU is a VM (Virtualization Machine) software application available in source code and compilable for a variety of Operating Systems. It is also available in the .tar.gz and .git formats. Qemu can also be used as an emulator, with the ability to run software intended for platforms other than the PC.
Images created with QEMU can be saved as .QCOW2; QEMU's own COW (Copy-on-Write) image format with support for AES and zlib.