Several factors can impact DNxHD MXF exports so there are a few things to check:
1/ Can you playback your construct in real time ?
2/ What is the construct resolution (no need for a 4K construct is export is 1080)
3/ Keep in mind the AMT used for MXF exports is CPU based. Your system looks ok but no info wrt the GPU used. Also Scratch v6 is....an antique compared to teh current v8.1
4/ To optimize: Pull and write from/to different volumes and ensure the Output node filter is set to Linear.