Large files/large entities perform bad in QCad. There are many dxf files that are created with thousand of lines that perform reasonable in other cad's/cam's that qcad cannot match.

One very useful tool, that could be added would help to improve qcad performance and at the same time enlarge the dxf quality (by replacing thousand of files by arcs), would be a tool called "arc fit" where, when possible and under a determined possible error margin (user selectable) creates arcs replacing the existing lines.
This would improve the file work in QCad also improves CAM software that would work after QCad, that I think is important for many users.

For example, if it is needed to use qcad to generate toolpaths for letters, after exploding letters, the output is almost impossible to work due to the number of lines that are created. Having arcs instead of lines would greatly improve qcad use.