The dual-module High Temperature gas-cooled Reactor Pebble-bed Module (HTR-PM) demonstration reactor, led by the Institute of Nuclear and New Energy Technology of Tsinghua University, has achieved successful operation. To enhance economic efficiency, the HTR-PM600S incorporating six nuclear steam supply system (NSSS) modules has been established as the next-phase development objective. The complex coupling effects of multimodule systems necessitate systematic simulation to preinvestigate operational characteristics. The simulator, leveraging its full-scope simulation capabilities and real-time computational performance, has evolved from an operator training tool to a critical operational analysis platform. By integrating neutronics, primary/secondary circuit thermal-hydraulic models, and control systems, the HTR-PM simulator establishes a complete framework that plays an important role in nuclear power plant operational studies. When extending the HTR-PM simulator to HTR-PM600S modeling, direct replication of standard NSSS module configurations substantially increases simulation time cost. Particularly in the primary circuit thermal-hydraulic model, the linear expansion of the number of thermal component network matrices and the dimensions of the helium flow network matrix have driven single-step simulation time beyond the 100-ms time step, constituting the critical bottleneck for multimodule simulation. This study dissects the computational time cost of different simulation tasks and matrix characteristics of a helium flow network, proposing targeted optimization strategies: implementing multithreaded parallel computing for thermal component networks and adopting KLU sparse matrix solver for helium flow network. Through these optimizations, the primary circuit thermal-hydraulic computation time has been reduced from 160 to 45 ms, achieving real-time performance while reserving an over 50% temporal margin for subsequent complex model expansions.