Content Jumping when Tabby Tab first loads

I would need to see the page on your site to be able to investigate the cause of this.

https://dev-uletters.com/samples/ (I will take down the maintenance plugin for a while as this is a development not public site)

This is probably due to your site loading 17 stylesheets, resulting in the loading of the tabby styles being delayed.

You might find that your styles load faster with a plugin to aggregate the CSS and load it in the head. I have had success with autoptimize https://www.ads-software.com/plugins/autoptimize/