Thank you very much!

As you have noticed, the main problem was in while loop of the boss thread. When I added 0,1 second delay and ran script with 50 threads cpu was about 6-8% and load -- only 0.3-0.4. The speed was more than 2000 urls per minute. The speed with $max_thread = 100 was 3900 per minute, more than I have expected (cpu = 12-16%)!

Many thanks for helping me!