Microdol-X is a Metol only developer. Such devs give very fine grain, but need a stop more light. All thes developers don't last very long if dissolved -the is no redox system which may protect the Metol.
Other Metol only devs are Perceptol or Rollei RLS. RLS comes in fluid form and keeps not very long even in unopened bottles.
It's not the Metol that's causing the speed loss it's the lower pH and restraining effect of the relatively high level of Sodium Chloride.