Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Dwill]

Shockwave and Aerial Slash should have shorter recast time than actual attack so your garuda dps should increased by those skills. During raging strikes you should use 2 festers + energy drain in case you will need mana later. By gcd i mean that you should time more your festers during you apply bio like in 1:06 you use fester and bio have 6 sec time left. You could wait that 6 sec and then use bio + fester at same time. And i never seen that kind of opener you use bio -> rouse -> spur -> miasma. You can use those abilities as off globals between instant casts.

Energy Drain should also never be casted during the initial Raging Strikes, it just result in a damage loss due to having one less Fester and early on, the additional MP will just be overkill. Even later on during an encounter, Energy Drain should be used when you're starting to get low, not when you are in Raging Strikes.

Ah yes, I see. Although my opener is actually quite good I would say, it bursts very high every time. Although I am still more fond of the idea to use raging, apply dots and reapply at the end of raging so you have a lot of dot time with RS. Good hint on the bio / fester, though. I'll take that with me. Cheers mate

Opener should be:

(Pre-pull: Raging Strikes > Intelligence Potion -> Bio II (Start pre-casting when your tank runs in)

(Pull): Bio II finishes > Miasma > Bio > Queue Contagion > Fester then (Make sure Bio is applied to not lose a full potency Fester) > (Miasma 2 if MP allows it(Short encounters)Ruin II > Swiftcast > Shadowflare > Rouse > Ruin II > Spur > Ruin II > Fester > Ruin II > Enkindle > Ruin > Ruin > Ruin II > Fester > Ruin II > Aetherflow

Then use off GCD during the encounter whenever you think MP will allow it and refresh DoTs when they have 3 or less seconds remaining. Energy Drain when your MP starts to dip too low.