How can some really large services (like Dropbox) afford to use Python as a primary language, if it’s one to two orders of magnitude slower than other, compiled languages?
Because raw language speed often doesn’t matter that much. In the case if Dropbox the client software spends most of its time waiting for bits to load from the network or from disk. Most large websites spend their time waiting for the database. You can’t speed up network or disk performance by using a faster language.