Hey, changing culture multiple times causes my website to lag for 20-30sec. I've figured out that there must be an issue with loading up a video. (When i delete the video problem's gone) The video mentioned before is a background video that is mutual for all the cultures, hence the vary by culture option is disabled. This is the way i change cultures:

<div class="header__languages">
                <a class="@(currentCulture == "pl" ? "header__languages--active" : "") header__language"
                    href="@Model.Url("pl", UrlMode.Absolute)">PL</a>
                |
                <a class="@(currentCulture == "en-US" ? "header__languages--active" : "") header__language"
                    href="@Model.Url("en-US", UrlMode.Absolute)">EN</a>
            </div>

Are you loading the video from the backoffice media section? If so, you may be running out of bandwidth. We usually advice against hosting videos in the CMS and instead recommend hosting videos on YouTube or Vimeo.