Tumors which are benign (non-cancerous) do often grow slow. The meaning of cancer, however, is rapidly dividing cells. These are the tumors which grow fastest. Other than an abscess, which is a pus and fluid filled growth that your vet would have gotten fluid back from, cancers are the fastest growing growth there is. Unless the lab says this is indeed an abscess, it will likely require some type of removal regardless.
It is not completely hopeless, however. First, there are inflammatory masses which can also grow quite fast, which are a type of auto-immune problem and with removal can be completely cured. Also, certain types of cancers with successful removal, can cause no further problems for your kitty. There are also options like radiation treatment, even for tumors that are non-cancerous, and chemotherapy, but these are expensive and time consuming. If the biopsy does not give definitive answers, you may want to consider removal quickly regardless, as this is likely to continue growing and due to where it is, will get to a point where completely removal is difficult for the surgeon.
I wish you and your cat the best, XXXXX XXXXX me know if I can help further.
I have heard of ganglion cysts, but they are not generally diagnosed in our companion animals. Instead, if this growth or swelling appeared just after an injury, which you did not mention earlier, chances are much higher it is more along the lines of inflammatory mass. Any type of cyst with liquid filling can be managed by withdrawing the entire amount of fluid, and I'm curious why your vet chose to not do so. after doing so, an x-ray could have been taken for further diagnostic of what occurred, knowing an injury did occur just prior.
I am very sorry for the diagnosis, but you made a very smart move getting that amputation done a.s.a.p before any sign of metastasis. In my experience, in which we've dealt with this many many times, cats do amazing well with 3 legs. In fact, the majority of cats we've amputated don't seem to even notice a difference and are moving about upon waking up from anesthesia. It's more difficult in dogs with broad chests and narrow waists to distribute their weight differently, but cats, being much closer to the ground already and having amazing sense of balance, rarely have a problem. A couple I have seen avoided moving much about 2 days post-op, but that was more likely to being a bit out of it from the amount of pain meds on board, and started slithering soon after, then walking about with no problems. I've never seen a cat not able to move about ever after an amputation, so don't worry about that.
While you'll likely have some future metastasis checks (chest x-rays, maybe an ultrasound far in the future), having had this done gave your cat an amazing chance at a well-extended life now, and I'm smiling to hear how quickly you acted on this and your cat's chances, which are extremely good.
You should feel very good about this now and I'm sure your cat will be recovering quickly from this.
Again, good luck with everything, and I'm still available on this thread should you need anything further.