Learn Arabic (Modern Standard): أي منفذ تستخدم، فهو سريع جدا.

They form an idafa (possession or construct state) relationship. When أي is used as a conditional word like 'whichever', the noun immediately following it must be indefinite and in the genitive case, making it منفذٍ (manfadhin).

What mood is the verb تستخدم (you use) in here?

It is in the jussive mood (majzum), meaning it takes a sukun on the final letter (تستخدمْ tastakhdim). This happens because أي functions as a conditional particle in this sentence, and conditional particles force the condition verb into the jussive mood.

Why is there a فـ (fa-) attached to the pronoun هو (it) in the second half of the sentence?

The فـ is required to link the condition to its result. In Arabic conditional grammar, if the result clause (the 'then' part) is a nominal sentence (a sentence starting with a noun or a pronoun like هو), you must attach فـ to the beginning of it.

Why does the result clause use the masculine pronoun هو instead of the feminine هي?

The pronoun هو refers back to the noun منفذ (port). Since منفذ is a grammatically masculine noun, the pronoun in the result clause must match its gender.