Learn Italian: Il servizio al bar è lentissimo. - The service at the bar is very slow.

The basic adjective for "slow" is lento. To make it mean "very slow" (the absolute superlative), you drop the final vowel and add the suffix -issimo. Because it describes il servizio (a masculine singular noun), it keeps the masculine singular ending -o to become lentissimo.

Could I just say molto lento instead of lentissimo?

Yes, molto lento means exactly the same thing. Using the -issimo suffix is just a very common and natural way in Italian to say "very" or "extremely" without needing to add an extra word.

Why do we use al before bar instead of nel?

al is the combined form of a (at/to) and il (the). In Italian, you typically use the preposition a when talking about being at or going to a bar, cafe, or restaurant. You would use in (and its combined forms like nel) for being inside countries, regions, or certain places like a bank (in banca).

Does the word bar change if we are talking about more than one?