Server respond is very long (25s). Is there a way to optimize the perfomance?
Let me try to turn this into a more valuable StackExchange for others who will find it, since the original poster apparently got help and solved his problem without participating here. We can see so on the linked site, which is now running as expected for Craft, quite rapidly.

What I'd like to do is suggest a less awkward way to approach his task. We can do that much more nicely due to Craft's actual abilities, and then there will be no need for index counters, full table scans of data, and so forth as in the original code. The site will then also be more efficient, offering something to the original question, while also having some convenient extra features.

The data involved is naturally hierarchic - Seminar 'plenums' with sessions within them. The sessions are run by speakers, whose information pages are linked. Speakers might be in more than one session, and it's possible that sessions would also be repeated in other plenums, so both should be independent data sets. By using Matrix variables, we can make it very easy to organize plenums, and provide per-plenum information such as times.

Thus in Craft terms, we should have a Channel of plenums, a Channel of sessions, and a Channel of speakers. To give the ordering flexibility, own times etc., and ease of relations, the plenums channel should have a Matrix variable for sessions scheduling. The sessions channel can even more easily use an Entries variable to hold details of the speakers for each session. Otherwise, the channels will contain simple Craft types for each of their data fields.

Such a design will replace the originally posted code with something like the following, which is much more simple and direct:

{% foreach plenum in plenums %}
  ...print plenum fields information
  {% foreach session in plenum.sessions %}
    ...print general session fields information
    ...print plenum-session-specific information, such as location and times
    &lt;a href="{{speaker.link}}"&gt;{{ speaker.speaker_name }}&lt;/a&gt; 
     {% if not loop.last %}// {% endif %}
  {% endfor %}
{% endfor %}


Note that highly efficient database queries will eliminate table scans and index variables, with the Craft-Twig loop variable supporting clean formatting of single or multiple speakers.

The Craft control panel will let us just pull around sessions within plenums to reorder them, while entering  any changed times or places very easily. Simple variations on this design would allow us as easily to change speakers when necessary for instances of a session, for example.

Since the sessions are only related to plenums, they can appear in multiple plenums just by adding them to the Matrix variable for each. Not much extra work would allow the Matrix variables to hold or over-ride date-times, locations, or even alternate speakers, etc., adding flexibility.

The original poster's site presentation design was appreciated as quite nice for its type, and this picture should show that Craft makes an appropriate data and code design for it just as nice and clean.