Why does my child write so slowly?
Almost never because they are being slow on purpose, which is the assumption most children in this situation have already had put to them several times. Writing looks like one skill and is actually four running at once: forming the letters, holding the spelling, deciding what the sentence will say, and keeping track of where the answer is going. If any one of those is still taking conscious effort, the whole thing slows down - and the child looks slow while working harder than the child beside them.
This matters because the fix depends entirely on which of the four is the bottleneck, and the standard response - more handwriting practice, more neatness drills - only addresses one of them. Applied to the wrong cause it does nothing, and applied to an anxious child it makes things measurably worse.
The rest of this post separates the four causes, gives you a way to tell which one you are looking at in about ten minutes, and sets out what to practise for each. If your child is in Class 4 or above and this is showing up as unfinished papers, that is the most common point at which it starts costing real marks.