Plug slowing down web site

Since I installed the plugin my website has slowed down massively, and although the photo sizes are a little big, the report that GoDaddy sent to me says that the main reason for the slow speed is “Remove query strings from static resources” and some of the examples of the pages are here:

https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/jquery.colorbox.js?ver=1.7.9
https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/themes/theme_1/colorbox.css?ver=1.7.9
https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/themes/theme_2/colorbox.css?ver=1.7.9
https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/themes/theme_3/colorbox.css?ver=1.7.9
https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/themes/theme_4/colorbox.css?ver=1.7.9
https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/themes/theme_5/colorbox.css?ver=1.7.9
https://desousa.photography/wp-content/plugins/gallery-by-supsystic/src/GridGallery/Colorbox/jquery-colorbox/themes/theme_7/colorbox.css?ver=1.7.9

Also, the report says “Specify a Vary: Accept-Encoding header” which again says that the Gallery is causing most of the problems. Can you help?

I’m not a web developer so I have no idea what the above means. All I know is that the speed report is saying the biggest reason the site is slow is due to the plug in.

website is desousa.photography.

Thanks
Dan

https://www.ads-software.com/plugins/gallery-by-supsystic/