Is there a maximum weight for spaying a cat, and what are the risks of fixing an older female?

For context, we have an appointment to get our 12lbs 8 year old female cat fixed in ~2 weeks. We are worried about her size and her age because she's technically overweight (she's also a small cat in general, so she's pretty chubby) and she was a rescue stray so she's likely halfway or more through her life. Has anyone had any similar experiences, or can any vets offer some advice? Thanks in advance.