Hey Denis,
I’ve looked into the Customizer problem and found a compatibility issue in the new versions of WP with a script that generates custom sidebars in the theme; added the necessary fix to your “functions.php” file (the 5 lines at the very bottom), now the Customizer works as intended. My apologies for the bug, and thank you for bringing this up!
Regarding the slider issue: you can try using website speed tests and similar tools to determine which resources are being used during the website load and how often.
Sincerely,
Andrii / Satori Studio