Why is adding log probabilities faster than multiplying probabilities?
Also, the Wikipedia page (https://en.wikipedia.org/wiki/Log_probability) is confusing in this respect, stating "The conversion to log form is expensive, but is only incurred once." I don't understand this, because I think you would need to take the log of every term independently before adding. What am I missing?


If you just want to compute $P(A_1)\ldots P(A_n)$ once, then you are right. You will have to compute $n$ logarithms and $n-1$ additions, whereas the naive method requires $n-1$ multiplications.

However, it is very common that you want to answer queries of the form:


  Compute $\prod_{i \in I} P(A_i)$ for some subset $I$ of $\{1, \ldots n\}$.


In that case, you can preprocess your data to compute all $\log P(A_i)$ only once, and answer each query by doing $|I|$ additions. 


  Finally, the justification that "computers perform addition faster than multiplication" is kind of vague. Is that specific to the x86 instruction set, or is it some more fundamental trait of processor architectures?


This is a broader question. In general it is (probably?) harder to compute multiplication than addition. Computing $a+b$ is linear in the size of $a$ and $b$ (using the trivial algorithm), whereas we currently don't know how to compute $a\times b$ with the same time complexity (check the best algorithms here).

Of course there is no definitive answer: for instance if you deal with integers only and you multiply by powers of $2$, then you should rather compare shift with add operations.

Nevertheless this is a reasonable statement on all common computer architectures: multiplication on floating-point numbers will be slower than addition.