I can read arabic however very slow. I have an app on my phone where it has all duas and ziyara’s. Is it permissible for me to recite them in english rather than arabic? As i will understand what I'm saying and also I can read long duas fast and they will be understandable.

I am also going arbaeen this year inshallah, is it permissible to recite ziyara aashura or ziyara arbaeen in english rather than arabic?

Yes you can read it