Microdol-X loses film speed, so it is a poor choice if you are looking for high film speed. Used straight, it takes the biggest bite out of the speed, as much as a stop or more.(posted 8670 days ago)This happens because Microdol-X as it dissolves some of the silver halide particles during development, it softens the edge of the grain. But it is the grains themselves that gives speed, so it's taking away speed as it takes away the hard edge of the grain pattern.
Used at 1:3, the speed loss isn't as great, because it doesn't dissolve as much grain.
For highest film speed, use a high accutance developer, like Xtol (which seems to have a real increase in speed in addition to fine grain and decent gradation), Acufine, Beutler, FX-1, FX-2, etc. These will, however, accentuate whatever grain pattern there is in the film
In short, any film/developer combination is a trade off among speed, grain size, and gradation. You can pick one, and the other two will suffer.