The prompt for the question appears to be the pasuk in parashat Vaychi, that is, Bereishit 49:21.
נַפְתָּלִי אַיָּלָה שְׁלֻחָה הַנֹּתֵן אִמְרֵי שָׁפֶר
The idea that this refers to extreme swiftness of the person Naftali can be found in a midrash appearing in Sotah 13a, and the first part of the midrash is cited by Rashi on the pasuk. The gemara reads, in part:
They replied to him, 'The document is in the land of Egypt. Who will go for it? Let Naphtali go, because he is swift as a hind'; for it is written: Naphtali is a hind let loose, he giveth goodly words10 — R. Abbahu said: Read not 'goodly words' (imre shefer) but imre sefer (words of a document). Among those present was Hushim, a son of Dan, who was hard of hearing; so he asked them, 'What is happening?' They said to him, '[Esau] is preventing (the burial) until Naphtali returns from the land of Egypt'. He retorted: 'Is my grandfather to lie there in contempt until Naphtali returns from the land of Egypt!' He took a club and struck (Esau) on the head so that his eyes fell out and rolled to the feet of Jacob. Jacob opened his eyes and laughed; and that is what is written: The righteous shall rejoice when he seeth the vengeance; he shall wash his feet in the blood of the wicked.
That is, the words of the blessing to Naftali are interpreted, out of context, to mean:
נַפְתָּלִי אַיָּלָה שְׁלֻחָה = Naftali is very fast
הַנֹּתֵן אִמְרֵי שָׁפֶר = he will bring the words of the scroll
Nothing in this midrash indicates that Yaakov miraculously granted Naftali super speed with his blessing. Rather, the brothers, using Yaakov's words, are saying that Naftali is very swift, and therefore, he should bring the scroll.
Indeed, the other midrash, about the confrontation with Yosef, also based on the words נַפְתָּלִי אַיָּלָה שְׁלֻחָה, describes Naftali as very swift. This was not a result of Yaakov's blessing, for it occurred well before Yaakov's blessing. And other descendants of Yaakov, in that midrash, have other superpowers. For instance, the ability to shout and shatter all the teeth of the Egyptians; to roar and have chest hairs pierce through one's shirt like iron nails; to kick a boulder and have it shatter; to restrain Shimon.
If so, the easy answer is that Yaakov gave him that descriptive blessing because, of the various brothers, he in fact possessed that super speed.
By the way, Rashi on the verse (linked above) mentions this midrash as a matter of midrash, but takes the simple peshat meaning to be that if refers either to the quality of their land inheritance or to to future events regarding the tribe of Naftali. Thus:
a swift gazelle: This is the valley of Gennesar, which ripens its fruits swiftly, like the gazelle, which runs swiftly. אַיָלָה שְׁלֻחָה means a gazelle that runs swiftly. — [from Gen. Rabbah 99:12]
[he is one] who utters beautiful words: As the Targum renders. [See below.] Another explanation: A swift gazelle: He (Jacob) prophesied concerning the war with Sisera: “and take with you ten thousand men of the men of Naphtali, etc.” (Jud. 4:6), and they went there with alacrity. And so it is stated there with an expression of dispatching, “into the valley they rushed forth with their feet” (ibid. 5:15).