In my limited experience, it varies greatly in the U.S. (at least in the South) depending on whether you're talking about English or Spanish-language halls.
When I was a child 30 years ago there were 2 English congregations and 1 Spanish congregation in my hometown. There are still 2 English congregations after all this time but there are now 4 Spanish congregations.
The English congregations were typical in that attendance hovered around 70-80%. Attendance at Spanish congregations was always above 100%, usually 130% or so if I remember correctly.
Churches of all kinds, particularly those that are more controlling (and often have tighter-knit communities as a result), were good resources for newly-arrived immigrants who had very few friends and family in their new location.
Also, a lot of the growth came from JWs who relocated to the U.S from other Spanish-speaking countries. It showed up as growth on the stats, but really there was simply a lot of relocation taking place.