After rooting HTC Desire L you can overclock the processor which will speed up your device (but may eat a lot of battery juice). Installing a custom ROM is also a good idea ( ROM - the unofficial version of Android on your device). It may solve a lot of problems but is a very dangerous and not recommended operation.
You can also delete unwanted applications on your HTCDesire L. Use a backup app in order to back up applications in case you needed to restore it in the future. Remember not to delete any system applications!