Hello Jafar,
 
Please try to edit the file “app/design/frontend/ves_bestmart/default/template/catalog/product/list/upsell_owl.phtml” then change the code:
 

slideSpeed: <?php echo $interval?(int)$interval:’500′;?>,

To:
 

slideSpeed: 600,

Or any integer value which you want (from 100 – 10000)