user269: Let suppose an unrated player plays a FIDE rated swiss tournament where its opponent's ratings are e.g. 1300,2000,2100,2200,2300 and wins all of them, then his initial rating will be 2100 according to FIDE rating calculator and FIDE documentation confirms this:
If he scores more than 50%, then Ru = Ra + 20 for each half point scored over 50%

Here Ru is the (new) rating of the unrated player and Ra is the average rating of the rated opponents.
Clearly, this player performed much better than the calculated rating (any performance calculator admits that). And I believe many before me have seen such an issue and such a performance (or maybe slightly weaker than that) from an unrated player is not impossible since many just don't play rated games. I'm wondering what is the rationale behind this? Does it mean that FIDE wants players to play enough many games before they win any title? Or is there any other reason to not use performance for calculating the rating? Were there already some discussions about the initial rating calculation?

user4418: 
I'm wondering what is the rationale behind this?

The rationale is very simple. 5 games is far too few to get a realistic and accurate assessment of a player's strength. That is why a new player's K factor remains at 40 until either they have completed 30 rated games. Additionally it remains at 40 for all players under the age of 18 with ratings under 2300.
Here is the relevant section from the FIDE Rating Regulations effective from 1 July 2017:

8.56 K is the development coefficient.
K = 40 for a player new to the rating list until he has completed events with at least 30 games.
K = 20 as long as a player's rating remains under 2400.
K = 10 once a player's published rating has reached 2400 and remains at that level subsequently, even if the rating drops below 2400.
K = 40 for all players until their 18th birthday, as long as their rating remains under 2300.
If the number of games (n) for a player on any list for a rating period multiplied by K (as defined above) exceeds 700, then K shall be the largest whole number such that K x n does not exceed 700.

A K factor of 40 allows for very rapid changes in rating. Hence the last clause above which seeks to limit the rating change in any one month to prevent see-sawing rating.

user12629: I think FIDE doesn't want people to have excessively high initial ratings, as this may result in a player getting discouraged as they lose hundreds of rating points.
Performance ratings are particularly inaccurate when dealing someone wins all or most of their games, as compared to situations where someone is close to 50%. A performance calculator would take your scenario and give the guy a performance rating of maybe 2900-3100. But I don't think it's reasonable to look at those results and say that they're likely a world-champion level player.
I'll agree that in the scenario you proposed, a 2100 rating is far below what is deserved. If you drop the win against the 1300 player from that scenario and replace it with a loss to a 2400 player, the initial rating changes from 2100 to 2260 despite that performance being objectively worse. FIDE chose simplicity over accuracy in some ways; they use average opponent's rating without looking at the individual opponents when calculating initial ratings.
Also note that scoring 0/5 or getting an initial rating under 1000 will result in you simply not being rated. This has something of an inflationary effect on ratings, as a particularly bad first tournament may not be rated at all, while a particularly good first tournament will. Perhaps the limiting of how many points you can get from a good first tournament counteracts this.

269: <p>Let suppose an unrated player plays a FIDE rated swiss tournament where its opponent's ratings are e.g. 1300,2000,2100,2200,2300 and wins all of them, then his initial rating will be 2100 according to FIDE rating calculator and FIDE documentation confirms this:</p>
<pre><code>If he scores more than 50%, then Ru = Ra + 20 for each half point scored over 50%
</code></pre>
<p>Here Ru is the (new) rating of the unrated player and Ra is the average rating of the rated opponents.</p>
<p>Clearly, this player performed much better than the calculated rating (any performance calculator admits that). And I believe many before me have seen such an issue and such a performance (or maybe slightly weaker than that) from an unrated player is not impossible since many just don't play rated games. I'm wondering what is the rationale behind this? Does it mean that FIDE wants players to play enough many games before they win any title? Or is there any other reason to not use performance for calculating the rating? Were there already some discussions about the initial rating calculation?</p>
 4418: <blockquote>
<p>I'm wondering what is the rationale behind this?</p>
</blockquote>
<p>The rationale is very simple. 5 games is far too few to get a realistic and accurate assessment of a player's strength. That is why a new player's K factor remains at 40 until either they have completed 30 rated games. Additionally it remains at 40 for all players under the age of 18 with ratings under 2300.</p>
<p>Here is the relevant section from the <a href="https://handbook.fide.com/chapter/B022017" rel="nofollow noreferrer">FIDE Rating Regulations effective from 1 July 2017</a>:</p>
<blockquote>
<p>8.56 K is the development coefficient.<br />
K = 40 for a player new to the rating list until he has completed events with at least 30 games.<br />
K = 20 as long as a player's rating remains under 2400.<br />
K = 10 once a player's published rating has reached 2400 and remains at that level subsequently, even if the rating drops below 2400.<br />
K = 40 for all players until their 18th birthday, as long as their rating remains under 2300.<br />
If the number of games (n) for a player on any list for a rating period multiplied by K (as defined above) exceeds 700, then K shall be the largest whole number such that K x n does not exceed 700.</p>
</blockquote>
<p>A K factor of 40 allows for very rapid changes in rating. Hence the last clause above which seeks to limit the rating change in any one month to prevent see-sawing rating.</p>
 12629: <p>I think FIDE doesn't want people to have excessively high initial ratings, as this may result in a player getting discouraged as they lose hundreds of rating points.</p>
<p>Performance ratings are <em>particularly</em> inaccurate when dealing someone wins all or most of their games, as compared to situations where someone is close to 50%. A performance calculator would take your scenario and give the guy a performance rating of maybe 2900-3100. But I don't think it's reasonable to look at those results and say that they're likely a world-champion level player.</p>
<p>I'll agree that in the scenario you proposed, a 2100 rating is far below what is deserved. If you drop the win against the 1300 player from that scenario and replace it with a <em>loss</em> to a 2400 player, the initial rating changes from 2100 to 2260 despite that performance being objectively worse. FIDE chose simplicity over accuracy in some ways; they use average opponent's rating without looking at the individual opponents when calculating initial ratings.</p>
<p>Also note that scoring 0/5 or getting an initial rating under 1000 will result in you simply not being rated. This has something of an inflationary effect on ratings, as a particularly <em>bad</em> first tournament may not be rated at all, while a particularly <em>good</em> first tournament will. Perhaps the limiting of how many points you can get from a good first tournament counteracts this.</p>
