There seems to be a certain misunderstanding when using food. People seem to be advocating using spell speed food over det/crit food. There are some issues with this:
The only spell speed food I can find that isn't Sachertorte are Pineapple Ponzecakes (2-star) and Pastry fish (lvl40 recipe). However, both of these give spell speed, vitality and piety. Surely you have no use for piety as a black mage, so this reduces the value you can get out of these items. Next would be Sachertorte, NQ caps at +24 spell speed and +6 determination and HQ caps at +30 spell speed and +8 determination. BUT there's a problem with this.
To get the maximum value out of Sachertorte you need 600 spell speed, 450 vitality and 300 determination for NQ/400 determination for HQ. Remember: Food gives you a PERCENTAGE boost, rather than a flat boost and caps at certain numbers. Unless you have 600 spell speed AND 300/400 determination, you can't get the maximum value out of these items. Assuming you have 470 spell speed and 250 determination with i110 build*, you'll only get 23 spell speed and 5 determination out of it. Combined +28 secondary stat points (HQ food). Apkallu Omelette (HQ) gives +27 crit which is easily reached with i110 and gives some accuracy as well. Meaning you could swap out one piece for more crit, det or spell speed in exchange for 8-9 accuracy. (you won't get more than 9 accuracy out of this, even with the cap at +11). Assuming you can perfectly utilize this: Apkallu Omelette gives +26/27 secondary stats.
Conclusion: While Sachertorte DOES give you a minor increase (+1 secondary stat more, in fact), is it really worth the price tag? Even if you forget about the value. Spell speed might even give you less value if it is just out of the threshold reach (you get 0.01s GCD for every set amount of spell speed). Ergo, you may only get -0.02s GCD instead of -0.03s.
*rough estimate, values may vary a bit
Edit: Even when going for max spell speed build with current i110 gear. The maximum value you can reach is 577 spell speed. While you suffer from severe accuracy issues and you still can't get any decent value out of determination (262 with same build). i90 gear was mentioned before. But I doubt 10-16 more secondary stats outweighs 5 primary stats (assuming it's an accessory)
Edit 2: Although I mentioned you could swap out "pieces", most direct example would be swapping out the Evenstar Earrings for High Allagan Earrings. Any other combinations requires complete overhaul of the build.