How can i make terrain smoothening more efficient?

basically, i have this 10kx10k map with alot of uneven bits and i was trying to smoothen it out to add buildings and whatnot and when i try to smoothen/flatten the terrain i realized it was taking forever and was incredibly inefficient so i was wondering if anyone knows a tool or other way to flatten terrain more efficiently

i think you can make the smoothing brush bigger?

ive tried but it lags my pc alot and it tends to miss certain areas

if it misses certain areas, just try brushing that area a lot, and if your pc lags, not my problem…