Summoner need a lot management if the fight have downtime
Do you know why?
Let me tell you. What you wanna do once the boss disappears? You want your Phoenix/Bahamut to always be on cd. Sometimes you can not do that and the boss disappears 2 seconds before you managed to do that? How to avoid that situation? You have to stack SPELLSPEED. Every fight may require a different amount of spell speed so prepare to swap your gear all of the time.
After downtime, you want to spam those 700 potency summonses out and skip your gemstone attacks, because the average potency for Ifrit and Garuda attacks is less than 400(without swiftcast). You have to count how many attacks you can do before your Phoenix/Bahamut is up again. If you spam your skills too much, you may have to use ruin 3 and suffer potency lost again.
Another fact is that garuda attacks are relatively weaker once you stack more spell speed like for example if you have zero spell speed garuda have an average 383 potency attack and if you have 10% spell speed average potency will be 348 potency because spell speed does not affect recast time for garuda. More spell speed you have weaker those attacks become.