Alain,
in what way is the router start-up torque causing problems?
Are you noticing sideways movement within the Z axis as the router starts?
If so, it's likely that any movement during start-up will also be apparent when taking a cut, in even the softest material, due to a lack of mechanical rigidity. Chatter will probably be observed on the walls of a pocket or profile cut, along with a poor top finish on a surfacing operation.
What material are you using to mount the router to the Z axis, what material is used for the Z axis mounting carriage, what material/diameter is used for the Z axis rails and how are the rails attached to the Y axis?...along with...how is the router mounted - bottom plate only, or bottom plate & body clamp - how much backlash exists within each axis... etc. are all questions that point to the machine's rigidity.
A simple machine constructed from MDF/Ply is barely capable of producing precise results in aluminium, as is an aluminium machine when trying to cut steel. However, we build the simpler machine to make progressively-better parts for a better machine, to tolerances which are hard to reproduce in quantity by hand.
How do I know? I have a 48" X 24" X 6" MDF & Aluminium DIY CNC machine that has a little runout in the Y axis leadscrew, which is currently poised to re-make it's own inaccurate hand made parts to correct the runout, then make parts for the more accurate machine.
Anyway, I'm rambling somewhat...
Aside from machine rigidity, correct feeds and speeds need to be observed for the material being cut and the cutter being used. A two-flute cutter requires a different horizontal feed-rate to a three or four-flute cutter... in a similar way to that which prescribes that a HSS cutter requires a different feedrate to a TCT or STC cutter. Chip-load-per-tooth is the important factor.
..Back to your original quandry.
Start-up torque shouldn't really be a problem in a DIY machine...you normally start the spindle, run the G-code, then stop the spindle at the end of the cutting operation. Unless you are using a spindle controller card to process multiple M3/M4 & M5 commands, stopping & starting between Z moves.