Is this something that can be tweaked a little bit, or is it kind of fixed and based on how the stock sim does its thing?
I was departing for VATVenture yesterday in the rain; during startup it was a light drizzle and intermittent speed cleared the window fine, but by taxi it was a good downpour, but not excessive, and they were ineffective at any speed. It wasn't like they were being cleared and then covered again quickly, they weren't being cleared at all.
Fortunately it was a short stroll to the runway and the wind did the rest