Well I heard that vertices and triangles are the main things to cause lags, and it would make sense that the more parts the more data used, so replacing things as meshes could reduce lag
I had my own similar topic about this and people said to use mesh parts: Should I keep trying in removing useless vertices in my model? - #4 by PANDASonNOOB
Difference between meshes and seperate parts (on performance) - #2 by KdudeDev