Currently this is 20 sec which is quite a long time imo if this skill is central to your build but what surprises me is that there is no passive, no legendary, no stat, no glyph, no paragon (even on the golem board) that would reduce this time.
Also the paragon board, for golem but also in general, is just provides boring generic increases to damage, life and armor/resistances.
Why not some attack speed, cooldown reduction on its ability, faster respawn, thorns, chance to buff the necro or inflict a status effect on enemy, etc?