Summoners and Spell Speed

Hi, new to Summoner at 70 and working hard on learning the job. I'm not a raider but I want to be comfortable doing standard content without feeling I'm holding my group back. So reading lots of guides etc. I'm coming across quite a few mentions of spell speed being the way to go (from what I understand this is to get as many wyrm weaves as possible) and that makes sense. But all my friends are telling me this is nonsense and I should avoid spell speed as it will cause me to clip when double weaving ....

Halp!

As you've said youre not a raider, I wouldnt be concerned with clipping while double weaving.
Raiding is about optimising every second you have, and losing a GCD has a significant effect. Outside of raiding however, youre much more likely to reap the benefits of having quicker cast times, giving you the freedom to react to things and make decisions on the fly quicker.

Spellspeed is not the way to go, it does not buff any of your cooldown or ogcd based damage, and summoners have the weakest valued gcd on average in the game at 120p for ruin 3. Even if you get lucky and that faster gcd nets you an extra cast, 'only' getting an extra ruin 3 is pretty bad payoff compared to what other jobs can do with their gcds. It's not just a matter of it making double weaving harder or not, that's just a 'bonus' downside, even before all that is taken into account, it's just a plain bad stat.

It is true that a faster gcd makes it easier to get 11 wyrmwaves per bahamut vs 10, but that's more a curiosity than a real consideration.
1. It's not a guarantee that a slower gcd can't get 11 wyrmwaves in the same setting
2. You can force 11 wyrmwaves anyway in any situation with an extra ogcd in bahamut (addle/tri-disaster)
3. Even if you did get an extra wyrmwave, you'd need to consider it against the dps loss of the crit/det/dh you lost to get that spellspeed.

In short it's a terrible stat, the speed stats tend to be bad for most jobs in the game. Generally speaking, you should run far away from any items with spellspeed on them.

Spell speed is the weakest substat for SMN. Generally speaking, the more a job relies on oGCD for big damage, the less desirable spell speed becomes, unless more speed also means more oGCDs (SAM).
On SMN, you want to minimize spell speed and maximize crit. A little sps won't kill you, but on SMN it has a very limited utility. Whatever guide is suggesting you stack sps on SMN, throw it away.

The substat priority for SMN is Crit > DH >= Det > Sps, with the goal of minimising sps as much as you can without compromising your ability to perform 11 wyrmwave Summon Bahamuts - with or without Addle being used. That is what's recommended by our guides over at The Balance (including Kodas and Imouto's).

We minimise sps because we gain the least from it by a significant margin due to Bahamut's raw damage, most pet actions and our oGCDs not benefiting at all from it but also because our Ruin GCDs are fairly weak. Sps does scale DoT damage but by itself is not enough to swing it to a stat we want to seek.

Clipping is also a legitimate concern but it all depends on your latency. You will never have so much sps that you clip on single weaves, however, double weaves are where clipping will vary with latency. That said, apart from the opener the job is not as intensive with double weaves, as such, you can worry much less about clipping your gcd.

If you want to set yourself a gcd limit, go no further than 1316 sps (2.35s gcd) as this is the general range your gcd is rolling too fast for Festers. A comfortable range is between 2.48s to 2.39s however your preference may vary. While you don't raid, you can build towards these BiS lists and focus on the tome pieces and melds they have:

i400 2.48 gcd
http://ffxiv.ariyala.com/1770B

i400 2.45 gcd
http://ffxiv.ariyala.com/1771J

i400 2.43 gcd
http://ffxiv.ariyala.com/1772A

Hopefully there will be some solid pieces from the final 24 man when it is released!