To see why: when you type "x.foo()" we need run type inference on the complete data flow chain that produced the value "x", so that we know which particular "foo" you're using. Throughout this analysis we may also need to know a lot about the python libraries you're using, since you may be passing values into and out of arbitrary third party libraries. If each of the steps in this chain triggered an SSD read then you'd often have a multi-second lag between hitting a key and seeing the result.