long mesh light map resolution vs multiple parts mesh and UV problem? or ...

I have 12m continuous floor mesh receiving shadows reflections needed in high quality seen in camera in one chunk.
To heave non zigzag line off shadow I increased light map resolution for mesh to 4096 but its slowing fps for real-time.
then I spilite to smaller meshes but now visual of surface has cuts. How to avoid it (heard something about UV’s manipulation ? but I’m not so advanced in UV problematic)

anyway what is best solution for good quality texture and shadow and fast fps real-time?

pleas advice

I did fast light recalculation at low quality however now I have plenty of this !