I downloaded the plugin but could not find ant documentation on the site or in the zip file so am a little disappointed.
First of all, the documentation is included in the most reasonable place it could be! Inside the plugin's parameters page. Access this page by going to Mambots >> Site Mambots >> Simple Image Gallery Plugin. If you cannot access the "mambots area" and get a "restricted access..." message, it's not a plugin error, it's a Joomla bug. Solution for that can be found here:
Unfortunately, I noticed a decrease in speed on the site. "Not to worry" I thought, it's not to bad.
As this a new site and still under development, I was adding more features. The site gradually ground to a halt.
I contacted my ISP who told me that the MYSQL queries this extension was generating was using 80% of the server processing power. Not only did it crash my site, it brought down the server and the other 450 sites hosted there. In the end, I had to remove the component and find an alternative solution.
They also told me they had this problem before with another site using 404sef
When using JoomSEF for content heavy site a special care need to be taken about its configuration as otherwise it can really become a problem. If you are not sure about how to configure, contact authors.
But at least 2 things to remember: Joomla! caching does a lot + switch off SEF for dynamic non-supported elements that generate too many links such as calendars and similar.