The laggier you are, I think the more skillspd helps. Your normal GCD skill activation shouldn't be affected by lag - as such the faster you complete them, the more amount of time you have to complete your mudra, activate it, and continue to reapply your debuffs and DoTs without dropping or clipping them.
The default GCD without skillspd under Huton is 2.15s, so with 2.0s, each GCD skill you will save 0.15s.
If you quickly refer to the 3:28 parse I've shown earlier, I have used 98 GCD skills. A quick math would tell you that I have saved 14.7s in this 3:28 parse. 14.7s is enough for 7 GCD skills. You can try a parse where after your opener, stay still and just auto attack for 14.7s and continue your usual parse and see how much DPS you have dropped.
Also, for 98 GCD skills with each 2.0s, it should take me a total of 3:18 to pull them off. However, the parser shows you that the total time of my run was 3:28 (I manually ended it, so no extra seconds counted). This means that the Ninjitsus I've used in this run ate up 10s outside the GCD cooldown.
In the list of skills used, you can see that I have used 5 Raitons and 3 Suitons. Also, what is not shown is 2 recast of Huton. So 10 Ninjutsus here. 10 Ninjutsus, 10s extra. Each Ninjutsu took 1s extra, which means my cast time of the 3 signs + activation is a 3.0s itself.
With my extra skill speed, I have bought myself 14.7s to cover up for the 10s I've used outside the GCD timer to use my Ninjutsus. I'm still in surplus of 4.7s over the default skillspd to cover up for any potential spikes and crap. If you did not had bought time for yourself and your Ninjutsu lags, you might find yourself that your DoTs and debuffs will start to drop, or you have to reapply them too early to prevent from dropping. These are all the things that wastes your DPS.
Sadly speaking, these are the little little things you have to watch. It is how Bards doesn't even have a rotation to begin with but there are so many bards out there that does crap DPS.