Slow convergence of GMRES with FMM assembler

Dear Bempp Community,

Thank you for this great package.

I am simulating sound radiation from a vibrating structure with around 120,000 elements using the Numba-only Docker image. To accelerate the computation and avoid memory issues, I switched to the “FMM” assembler. However, I’ve noticed that the GMRES solver converges very slowly. After a full day of computation, it reached around 1,600 iterations before I decided to stop the process.

My machine has 32 GB of RAM and an Intel i9-13900H CPU (2.6 GHz base clock). Is there something additional I should be implementing when using the FMM assembler to improve GMRES convergence? Could the issue be related to preconditioning?

Thank you in advance.