Why does bootstrap lag on mobile?

Question

JavaScript

All modal windows lag a lot when opened, is it a bootstrap feature or something wrong? Has anyone encountered this? I check on the tablet and phone - it's the same everywhere.
UPD

<img itemId="<%=items[i].id%>" data-toggle="modal" data-target="#zoomItem2" src="<%=items[i].url%>" alt="..." class="img-thumbnail small-img">



<div id="zoomItem2" class="modal fade " role="dialog">
      <div class="modal-dialog modal-lg">

        <div class="modal-content panel-primary">
          <div class="modal-header panel-heading">
            <button type="button" class="close" data-dismiss="modal">&times;</button>
            <h4 class="modal-title">Position <span id="big-photo"></span></h4>
          </div>

            <div class="modal-body panel-primary">
                <div class="row">
                    <div class="col-md-7 centered text-centered">
                        <img src="/images/noimage.gif" alt="..." class="img-thumbnail modal-img">
                    </div>
                </div>
            </div>

          <div class="modal-footer">
            <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
          </div>
        </div>

      </div>
    </div>

Answer 1

nothing lags there, if he didn’t break it

Answer 2

It also crashes on my phone. True, while I did not install it on a server, I checked purely laid-out html files. Some of the bootstrap styles were changed, like the standard colors, borders, etc.
Lags: navbar-collapse opening animation, panel (they are also accordions) lags. Didn't check the other one. On the other hand, everything is fine on the computer. Author, if you have solved the question, please answer. Thank you!