Writes do not directly impact ARC. The ARC, (Adaptive Replacement Cache), is for repeated reads.
You can see an extensive list values in ARC via;
arc_summary
Also, it helps to list your entire hardware setup, including disk make & model. And how the pool is laid out. Plus. which version of TrueNAS you are using. (You posted in the very old FreeNAS forums...)
Their are multiple common performance problems that new users can make. We can review your setup and see if any apply to you and your NAS.