My senior dog suddenly started eating very fast, why

A dog that has always eaten quickly is simply showing a stable personality trait, but a dog that changes pace abruptly in later life is showing a genuine shift in behavior, and genuine shifts usually have a cause worth identifying rather than just managing with a slow feeder bowl.

An increase in hunger can come from several manageable and less manageable causes in older dogs, and cognitive changes that affect memory of having already eaten are also seen in some senior dogs. None of these can be told apart without a vet examination and, often, some basic blood work.

Note when the change started, whether weight or thirst has also changed, and whether the dog seems otherwise well, since this information helps a vet narrow things down quickly. A slow feeder bowl can still make individual meals more comfortable while the cause is investigated.

A dog with joint or dental discomfort may rush through a meal specifically to spend less time in an uncomfortable standing or bending position, rather than out of genuine hunger, which is a less obvious explanation worth mentioning to a vet alongside the more commonly considered metabolic and cognitive causes. Watching whether the dog seems reluctant to stand for long periods, shifts weight between legs while eating, or seems stiff getting up afterward can all support a pain related explanation worth describing clearly during the vet visit.