Slow web site because of a module slide calling images in http instead of https

[FloreG]

Hello, 

I called my web host today to alert them that my website is uploading really slowly or showing "no page found"...

They told me that everything was working perfectly on their side. 

However, they bring my attention on plugin that could get the website slower. Indeed, my layer slider plug-in is calling  images in http instead of https. I don't really know what that means but apparently it's the reason why it could be slower. Actually the server finds some errors because the website is in https and the images are in http. 

Does someone know how to resolve this problem? Which folder should I update in my FTP? What should I write in the code? 
My template is JMS Meagamall if someone knows how to deal with it.

thanks in advance community

Flore