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'The first step where the carbonium ion forms is slow and because it contains only the halogen, the general speed of the reaction will be slow.'
I don't understand why should SN1 reactions happen slowly.
ps. Please be specific about the question, I am aware of other characteristics of SN1 reactions.
From what I understand from the quote, it's not saying that $\ce{S_N1}$ reactions occur slowly, but rather that if the first step of the reaction occurs slowly, it'll cause the overall reaction to occur slowly.
So, the $\ce{S_N1}$ reaction contains the two steps, the first one where the carbonium ion forms as your book says, and the second step where the substitution occurs. The first step is the slowest step of these two steps, whereas the second one is quite fast. The first step is thus called the rate determining step, meaning that it "controls" the overall rate of the reaction. If it is slow, overall the reaction is slow. If it is fast, then the overall reaction will be fast. At least that's what I understand from it.
