[boost] boost thread slow with only one thread

hi! here is my problem i am multithreading my program with boost but i don't get what i expect, and i can't understand why! i just run one thread and wait for it, and the fps are divide by 3. basically with no thread i have 30fps, and with 1 thread i have 10fps. here is the code: Code: boost::thread_group forces_threads; forces_threads.create_thread(boost::bind(&ElasticRods::computeBendingForces,this,edges,kb,edges_initial)); forces_threads.join_all(); i use thread_group because i want to run 7 thread but it is already too slow! do you have an idea of why this is so slow? thank you for your help!