You could lower Gi vs. AA balance, as that can help shift processing power onto DOF and MoBlur (see
https://coronarenderer.freshdesk.com/support/solutions/articles/5000526159-what-is-gi-vs-aa-balance- ). Is it just that it is taking a while to clean up, not that the effect is too strong (in which case, adjusting the camera values could help). Another thought I have (not tested this, so it is just a thought!) is that raindrops closer to the camera will show more motion blur, so perhaps limiting how close they are to the camera could help by avoiding ones that have excessive motion blur.
Last, there is also the option to use the CGeometry_Velocity and add the moblur in post (e.g. using the free Fusion, or commercial plugins for After Effects like RSMB), while this can be less realistic, it can certainly be faster, and may be just fine for something like raindrops. See