This paper presents a new performance and routability-driven routing algorithm for symmetrical array-based field-programmable gate arrays (FPGAs). The contribution of our work is to overcome one of the most critical limitations of the previous routing algorithms: inaccurate estimations of routing density which were too general for symmetrical FPGA. To this end, we devised new routing density measures that are directly linked to the structure (switch block) of symmetrical FPGA, and utilize them consistently in global and detailed routings. With the use of the proposed accurate routing metrics, we developed a new routing algorithm called a reliable net decomposition-based routing which is very fast, and yet produces excellent routing results in terms of net/path delays and routability. An extensive experiment was carried out to show the effectiveness of our algorithm based on the proposed cost metrics. In summary, when compared to the best known results in the literature (TRACER-fpga_PR and SEGA), our algorithm has shown 31.9% shorter longest path delay and 23.0% shorter longest net delay even with about 9 times faster execution time.