I did some tests in Firefox and Chrome with the extension and about 5 tabs activated. The extension is very light and doesn't use so much RAM (it use even less RAM than used by most of modern websites, tested with Chrome). If you use a previous version of the extension, update it (the latest is the 2.3, and I fixed performance issues in 2.1). You can contact me and provide more detailed informations about your issue here: https://github.com/Eliastik/page-shadow/issues or http://www.eliastiksofts.com/contact/

EDIT: I made some memory optimizations for the version 2.4.