A couple of things come to mind -
The 300D is certainly no speed demon in anything it does. However one generally doesn't notice this until you use a 20D or newer model camera...
The lens assortment you are using, while capable, is not considered fast in the focusing arena nor the light areas. The 50/1.8 is a 'fast' lens but focusing with it can be difficult and inconsistent. Patience with it can result in spectacular images.
Trying to remember my process when shooting the 300D - I believe I would focus on my subject and simply hold the shutter button half down, recompose, and shoot. On rare occasions did I feel the need to lock the exposure, then focus, recompose and shoot. Just keep in mind that every time you release and press the shutter button you get a new focus point.






