ImageCache is a module that can generate image thumbnails on the fly, to exactly fit your dimensions. The thumbnails is only generate once, so should not influence performance. The benefit however is to the end user: visitors will download smaller images instead of the full unscaled version.
In the earlier days of the internet often people would upload huge images scaled by setting HTML width and height. Images looked small, but took ages to download. ImageCache fixed this problem. Use ImageCache in combination with various tools, including inside node.tpl.php and page.tpl.php.
- Smaller images, reduced bandwidth
- Allows extra compression options