Okay in Dragon Quest if you fly Ramias from one point and cross the world back to that point it takes ~1.75 days. Now it is more likely that the Upper World in Dragon Quest III is the size of our world, but this is a lowball calc.
Now Ramias is magical and divine, IIRC one iteration of it is called the Great Godbird. So it probably is faster than normal birds. I will say he is as fast as the fastest bird, which can go to a speed of 200 mph
200 miles per hour * 1.75 days = 8400 miles
Now this is the circumference of the planet, its diameter is 2673.80 miles
This is the upper world, later in his adventure Erdrick found out about Torland via the pit of Gaia, a seperate realm made by Rubiss.
Here is a size comparison of Alefgard and Upperworld
The Upperworld is 4096 pixels wide
4096 pixels = 4080 miles
Alefgard is 2200 pixels, or 2191.40625 miles (So a 1436.1230 mile diameter)
Alefgard is actually a small part of the land Torland, this is the map
http://lparchive.org/Dragon-Warrior-II/11-DW2_M_Map.png
Malroth crossed Rhone in a span of ~20 seconds.(I miscalculated it earlier)
Rhone is a bit larger than Alefgard. I will say it is 1500 miles.
750 miles/20 seconds = mach 354.699815
Which is massively hypersonic
Now Malroth = Rubiss, and since Zoma actually defeated and sealed away Rubiss. He would have had to be at least in the massively hypersonic range too. Most of all Nokturnus, the god of destruction was FTE to Botsu (A guy who is faster than these characters anyways) so he would be MHS-MHS+ normally and probably Sub-rel at full power
Also Malroth was made to destroy Torland and Rubiss created Torland. So they can destroy this ~9931.1556610 miles span of continents. Would that make them 6-B, 6-A, or 5-C?