The speed with which students handle tests and other activities does not reflect on the quality of their thinking. In fact, taking longer to respond may be more effective than answering too quickly.
There are several possible explanations for why your child works slowly on everything -- it could be a habit, or perhaps, her mind wanders and she isn't even aware of it. Or she could work slowly because she is a perfectionist.
If your daughter has normal mental, physical, and emotional development, you should be able to help her speed up. This is especially true because she is beginning to find her lack of speed frustrating and is motivated to change.
You and your daughter need to choose a school task that she should be able to complete in 15 minutes. Then, have her consider how she will pace herself to complete the task. While doing the task, she needs to focus on completing it in the allotted time. If your child does not complete the task, she should keep trying until she can do it in the agreed-upon time.
Once your daughter speeds up on one task, choose others for her to complete. When the child sees that she is working faster, she will be motivated to keep on practicing to improve her response time.