Looks like your external hard drive is USB 2.0 read/write speeds which is pretty poor performance, it may be decent for 1 video stream at a time. You’ll want a drive with double those speeds for mixing multiple HAP videos without performance issues. Can you share your memory tab and are you able to use the latest version of VDMX? By the looks of it your VDMX isn’t doing much if it is only showing 18% CPU usage. Can you please take screen shots of your activity monitor while running VDMX with multiple video streams playing?
Also please run EtreCheck and let me know what the results are. I want to rule out that it is not a hardware issue.