A service is scalable if it results in increased performance in a manner proportional to resources added. Generally, increasing performance means serving more units of work, but it can also be to handle larger units of work, such as when datasets grow.1
Another way to look at performance vs scalability:
If you have a performance problem, your system is slow for a single user.
If you have a scalability problem, your system is fast for a single user but slow under a heavy load.