Why sure you can, and indeed you have just done so!

I really cannot imagine what it is that makes you think you cannot do what you have just yourself done. Do you mistrust your own eyes? What’s the source of your afraidness here regarding fast-pacedness?

Perhaps it’s just its nonceness or ad-hoc-ness, its brand-newness or its yuckiness, or even its unapproachableness, but those are all their own concerns unrelated to what you have created.

Why? Because -ness is particularly superpowered among English derivational suffixes, that’s why. It does what it pleases, as too should you. It is a suffix of neverending plentifulness, for its cornucopiousness knows no bottom!

I strongly recommend that you read Aranoff and Fuhrhop’s 2002 paper on Restricting Suffix Combinations in German and English: Closing Suffixes and the Monosuffix Constraint, published in the journal Natural Language & Linguistic Theory.

There you will learn that -ness is the most liberal of all derivational suffixes, one that freely combines with nearly any base, even atop Latinate bases or piling atop not merely Germanic bases but even other Germanic suffixes, two things that just never happen with other Germanic derivational suffixes.

That’s why you can have cluelessness but not cluenessless. Heck, you can even have admissibleness! It doesn’t even have to be one word, which is where anal retentiveness comes into the picture.

So do as you will with your fast-pacedness. Just consider it your superpoweredness shining through. Eventually your friends may tire of your shock-and-awe-ness, but don’t let them hold you back! Be creative!

The OED Speaks

The OED says that the twin etymons of paced the adjective are pace the verb combined with the -ed inflectional morpheme. The verb appeared in print a couple of centuries after the noun, which dates to 1300. The primary sense they give for the adjective is:

1. Having a particular pace, gait, or rate of walking or moving. Chiefly as the second element in compounds.
   
   For more established compounds, see the first element; recorded earliest in well-paced adj. 1, with reference to a woman metaphorically likened to a horse.

(Sense 1 is the one that applies here; their senses 2 and 3 are for other sorts of uses.)

Here is that earliest citation just now mentioned:

• ?1523 J. Fitzherbert Bk. Husbandry f. xxxiᵛ
  The first [property of a woman] is to be mery of chere, the seconde to be well pased.

As you see, spelling was not yet at all standardized during late Middle English. They then proceed to illustrate the historical progression of the dominant spellings used over many centuries, including in this order and beginning with the first just given: well pased, slowe paste, high-pac’d, slowly pac’d, middle paced, slow-pac’d, stealthy-paced, quick-paced.

Elsewhere in the OED they separately document all of broken-paced, easy-paced, fine-paced, firm-paced, giddy-paced, half-paced, heavy-paced, lazy-paced, lofty-paced, medium-paced, motor-paced, proud-paced, quick-paced, slow-paced, snail-paced, swift-paced, tandem-paced, thorough-paced, tortoise-paced, true-paced.

As you see, it’s used in really all sorts of compounds, many of them rather more unusual that you own. Honestly, given quick-paced and swift-paced, not to mention slow-paced and tortoise-paced, what can possibly be “wrong” with your fast-pacedness? It lacks but a -ness, which is covered in the section previous to this one and in the section subsequent to this one alike.

Their most recent citation included under the paced headword is ideally paced, which is an open compound:

• 1990 Gramophone May 1982/2
  A buoyant, ideally paced scherzo whose refulgent second subject Litton pointedly relishes even more on its second appearance.

My personal conclusion, based solely on OED inspection alone, is that your own version is unremarkable, no matter whether viewed diachronically or synchronically.

Published Citations of -pacedness

Here for your delectation are a couple of centuries’ worth of references to published instances of various sorts of X-pacedness terms. I see no difference whatsoever between swift-pacedness or slow-pacedness and the asker’s fast-pacedness.

• 1823 European Magazine and London Review, Volume 84, p 141.
  This thorough-pacedness has communicated itself to his pleasures as well as his labours; and he runs through the business of relaxation with earnestness, because he has deliberated and resolved upon it.

• 1836 Isaac Taylor The Characters of Theophrastus, p. 77
  In this movement there is that peculiar character of even-pacedness which belongs to all the undisturbed operations of nature.

• 1884 L.H. Grindon Life: Its Nature, Varieties, and Phenomena, p. 397 Virtue, wisdom, poetry, the Bible, are matters which from intellectual slow-pacedness, or moral disrelish, excite only moieties of interest, but life is the central, universal, indomitable solicitude.

• 1892 The American Magazine, Volume 34, p. 349
  Scientific development has invested most things with a modern air of improvement and utility that contrasts violently with the staidness and slow-pacedness so characteristic of the age of our grandfathers.

• 1903 E.V. Lucas The Works of Charles and Mary Lamb: Letters, 1796-1834, p. 14
  Your simile of the Laplander..will bear comparison with any in Milton for fullness of circumstance and lofty-pacedness of Versification.

• 1983 S.G. Abbas The Immortal Poetry & Mir Anis, p. 174 Very like the presentation of the qualities of the sword, Anis made a description of the grace, faithfulness, agility, trot and swift-pacedness of the horse.

• 1985 J.M. Heath and M. Payne Text, Interpretation, Theory, p 170
  ..to Tennyson's use of “the most characteristic of the Keatsian metonymies, which is the substitution of a near-stasis or slow-pacedness for the language of the sense, for the sounds and sights of passing time.”

• 1985 Peter Clark A Review of the Theories of Time and Structure for Organizational Sociology, p. 31
  Probably the other most important dimensions requiring investigation are societal expectations about the even pacedness of working and excitement control (Elias, 1982).

• 1994 Industrial & Labor Relations Review Volume 48, p. 335
  We then ran preliminary regressions with salaried status as the dependent variable and a “pacedness” variable (combining the three measures of pacedness into one) as one of the independent variables..

• 1999 K. Jahandarie Spoken and Written Discourse: A Multi-disciplinary Perspective, p. 133
  In his Dialogues on the Differences Between Speaking and Writing, Pedro de Navarra, the 16th-century Spanish linguist, referred to several such differences, including the evanescence, contextuality, other-pacedness, and redundancy of speech vs. the permanents, autonomy, self-pacedness, and conciseness of writing

• 2012 M.S. Frings Philosophy of Prediction and Capitalism, p. 3
  This slow-pacedness of philosophical thinking does not, however, agree very well with the restless speed of our lives and present-day cravings for ever faster short-term solutions of all impending problems.

And although these terms may not be especially popular for any number of possible reasons, they are certainly morphologically grammatical and can be found here and there in various published works from the past 200 years.

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^{Post script}

No dictionary can ever tell you something is not a word!