Think about it this way: Speed is the number of requests/second a single server can process.
Scalability is the amount of computing power you lose when splitting a job amongst multiple machines.
Intuitively you can think of it like asymptotic complexity. You can have a really fast implementation of quick sort, but at a large enough input size it will be flattened by a very slow version of radix sort.
Similarly you can have the fastest web server on the planet, but if you scale poorly, at some number of nodes even the slowest albiet well scaling system will out perform you.
Think about it this way: Speed is the number of requests/second a single server can process.
Scalability is the amount of computing power you lose when splitting a job amongst multiple machines.
Intuitively you can think of it like asymptotic complexity. You can have a really fast implementation of quick sort, but at a large enough input size it will be flattened by a very slow version of radix sort.
Similarly you can have the fastest web server on the planet, but if you scale poorly, at some number of nodes even the slowest albiet well scaling system will out perform you.