Tzav. Three He'aros on Zerizus

 1. Rashi in the beginning of the parsha:

צו את אהרן. אֵין צַו אֶלָּא לְשׁוֹן זֵרוּז מִיָּד וּלְדוֹרוֹת; אָמַר רַבִּי שִׁמְעוֹן, בְּיוֹתֵר צָרִיךְ הַכָּתוּב לְזָרֵז בְּמָקוֹם שֶׁיֵּשׁ בּוֹ חֶסְרוֹן כִּיס (ספרא):

Rashi in Devarim 1:16:

ואצוה את שפטיכם. אָמַרְתִּי לָהֶם הֱווּ מְתוּנִין בַּדִּין – אִם בָּא דִּין לְפָנֶיךָ פַּעַם אַחַת, שְׁתַּיִם, וְשָׁלוֹשׁ, אַל תֹּאמַר כְּבָר בָּא דִּין זֶה לְפָנַי פְּעָמִים הַרְבֵּה, אֶלָּא הֱיוּ נוֹשְׂאִים וְנוֹתְנִים בּוֹ (שם):

Rav Bergman (Ma'amarim here in Tzav) points out that Mesinus in the context of din means moving slowly - as in Brachos 20a,

כי הא דרב אדא בר אהבה חזייה לההיא כותית דהות לבישא כרבלתא בשוקא סבר דבת ישראל היא קם קרעיה מינה אגלאי מילתא דכותית היא שיימוה בארבע מאה זוזי א"ל מה שמך אמרה ליה מתון אמר לה מתון מתון ארבע מאה זוזי שויא:

Rashi there:

מתון מתון - לשון מאתן:

ד' מאות - ב' פעמים ב' מאות כלומר השם גרם לי לשון אחר מתון מתון לשון המתנה אם המתנתי הייתי משתכר ד' מאות זוז:

So what does Tzav connote? Alacrity or deliberation? Rav Bergman answers that there is physical atzlus and there is mental atzlus. A person that is mentally lazy will answer questions by rote. A person that is a mental Zariz will think through the question and the circumstances and quickly review the basis for the psak.

So Zrizus is always speed and alacrity. But in the case of psak, it means to be mentally agile and quick, and to use every iteration of a question as an opportunity to reexamine old certainties.

2.  From Reb Yerucham.  The word tzivui was used in regard to the exact same dinim in Tetzaveh. Two ziruzim were needed, exactly as the Mesillas Yesharim says in perek 7. I am widely known as something of an expert on atzlus, so trust me when I tell you that the Ramchal is right. Sometimes you decide something needs to be done, and somehow you congratulate yourself on your strength of character as demonstrated by your decision, and the self-congratulatory satisfaction replaces the actual motivation to get it done. The same happens when you begin a project that takes time and work, and after a day or two..... 

3.  Reb Moshe, in the new Kol Rom, says that the instruction of zerizus is particularly important in the parsha of Olah. A person brings an olah, which seems to yield very little practical benefit to anyone, and it reminds him that our efforts are not tied to our success. The Ribono shel Olam decides what will succeed and what we will have. A person might use this faith, this sharp spiritual perception, to justify sloth. So the Torah says No, whatever you choose to do for a parnassah, don't do it half heartedly. Do it with energy and diligence and focus - with Zerizus!

(You need to see it inside, but I can't quote it verbatim because A, it's copyrighted, and B, even if I decided it's ok to steal someone's hard work, it's not available online.)