Any .dmg file is a DISK IMAGE, it cannot run unless you decompress the disk image and mount it to the Finder, and then open and execute the files that are in the disk. It is not an executable program or application file that can do anything at all.
Anything in it that is “Infected” is not going to affect your computer as it is just a stored file until it is unpacked, mounted as a disk, then the apps installed, and then executed. That cannot happen until YOU as the user, do it. It certainly cannot affect the speed of your computer.
If there was an infected file found by your anti-virus in that “nexdefinstall.dmg” file, it is the equivalent of finding a virus in an email package that has yet to be opened: it’s advisory only. . . and don’t mount that disk image.