The performance of the OpenBlas library decreases when running in multi-threaded environments

hello，
C++programs run on Ubuntu 24.04 using the OpenBlas library. The function execution time is less than 1ms on a single thread, but it increases several times on multiple threads, and the system call time also increases.
The openblas library is installed using the apt tool, and the default settings are as follows：
OpenBLAS 0.3.26 NO_LAPACKE DYNAMIC_ARCH NO_AFFINITY Haswell MAX_THREADS=64

The single thread time is less than 1ms, and opening 8 threads can achieve a maximum of 10ms and a minimum of about 1ms.
The thread function contains a loop traversal operation. Delete the following line of code, and the multi-threaded performance is similar to that of a single thread.
arma::cx_mat pmusic = ss * nnn_md * ss.ht();
Among them, nnn_md is a complex matrix of 15 * 15，ss is a complex matrix 1*15
then,how to solve the problem of multi-threaded performance degradation？
int main()
{
openblas_set_num_threads(1);
int current_threads = openblas_get_num_threads();
printf("current thread num is %d\n", current_threads);
for (int i = 0; i < 1;++i)
{
std::thread t1(processfunction);
t1.detach();
}
while (1)
{
}
}

Start by reproducing the supplied C++ example on Ubuntu 24.04 with OpenBLAS 0.3.26, including the complex matrix expression and the reported thread settings. Compare single-thread and multi-thread timings and system-call time; done means identifying the cause of the degradation or documenting the conditions and configuration needed to avoid it.