Learn Polish: To urządzenie ma wiele funkcji, jednak działa bardzo wolno. - This device has many features; however, it works very slowly.

Jednak (however) is perfect here because it introduces a surprising drawback or a direct contrast about the same subject (the device). Natomiast translates closer to "whereas" or "on the other hand." While you could technically use it, natomiast is much better suited for comparing two different subjects (for example: "This device has many features, whereas that device is very simple").

Can I change the position of jednak in the second half of the sentence?

Yes, jednak is quite flexible in a sentence. Placing it right after the comma is the most common and neutral choice, but you can also place it after the verb to give the sentence a slightly different rhythm: ...funkcji, działa jednak bardzo wolno.

Why is the word for features written as funkcji?

It is in the genitive plural case because it directly follows the quantifier wiele (many). In Polish, words that express an amount or quantity (like wiele, dużo, or mało) force the noun that follows them into the genitive case.

Why does the sentence use działa for works instead of pracuje?

In Polish, pracować (to work) is used when talking about people or animals doing labor or having a job. When you are talking about machines, electronics, or systems running and functioning, you must use the verb działać.