Suggestion:
First: You should try to write one at a time. I know it's hard. You need time to get used to it. After all, it may be slow at first. It doesn't matter, as long as you start, it won't be too late.
Second: If the improvement effect is not obvious, you can compare your pen-holding style with other students to see if your pen-holding style is standardized, which is also influential.
Third: if you really can't change it, work hard on the answer format, simplify the questions and write them in the first row, then mark them with numbers and pull them to the font spacing so that the teacher can see them clearly as much as possible.