I'm not sure of the context of the question:
When you say "writing", are you referring to the act of putting pen or pencil to paper, and then making the necessary movements of the relevant hand to form a kind of text proxy?
... Or are you referring to the process of transmitting thought patterns into text form, whether via pen or keyboard?
The physical process of writing causes me issues, in that I cannot do it quickly, and must always ensure that the letters are clear, sharp and concise... but expressing myself in text online is something I have been doing fluently for well over a decade now, and have absolutely ZERO problem with.
And while I wasn't exactly high-scoring in my English exams at GCSE level, I didn't score too low either (B for English, C for English lit)... as some things I was better at than others, clearly (and I rarely do well at things I don't like, as I tend to defy them).