Long story short, 10 yr old mare went in for a lameness and has OA in hocks; not super worried about it - we injected and went on our way. Now I'm wondering about long-term care. Vet recommended looking into osphos to slow the fusing process, though they looked pretty fused already. I've also heard about people trying speed up the process because it's not painful once it's over (correct me if I'm wrong). So, are there pros/cons to speeding up the fusion vs slowing down the fusion?? I've read A LITTLE about both, but haven't dealt with fusing hocks yet, so now I want to learn and make the best decision for my horse. Educate me!!

I've never heard of a vet wanting to slow down the fusing process... like you said it's better once they're fused. I had a horse that was dealing with arthritis and never completely fused. He was getting injections once a year then every 6 months then that wasn't even helping. A friend/trainer suggested TLC's Equibone. It's expensive (not as much as injections) but it was worth it! It helped him soooo much!! He finally was able to move properly without pain and no injections. Once they were fused I was able to take him off of it and he was still good. I highly recommend it. 

You want to speed it up. Inject your horse when needed and keep riding. If it doesn't completely take away all the pain, you can also give Equioxx if needed.

Plus.... my vet told me Osphos could help SPEED it up (not slow it down?)

I can understand giving Osphos to help with the bone pain, but wouldn't want to slow down the process. I am currently going through the same thing, Osteo-MAX by 100X Equine has helped mine tremendously. 

She may have just been having a difficult time explaining it in layman's terms so that I could understand, and I more than likely misunderstood what she was trying to tell me lmao

Did you have trouble getting your horse to eat that?  I ended up putting it in a syringe and squirting it in my guys mouth, he acts like I'm killing him, lol.  He absolutely won't eat it, good thing it's easy to give that way, no mess or mixing.

Osphos won't necessarily slow the fusing process down but a might very well build a better bridge without as much boney reaction . 
inject as needed, don't wait, keep riding. Previcoxx if you think she needs it. 
once she fuses, she shouldn't need any pain meds . 
The time it takes varies per horse, some never do actually finish fusing. 

My horse is 18 and his hocks never fused. We have been doing injections to speed up the process. From my understanding, you definitely want them to fuse.