I did other tests. Now the maximum speed between node1 (client) and node2 (server) is exactly 338 Mbit/s (never over that). And during the test, tincd on node2 is 100% CPU. This is not normal, because node2 CPU is more powerfull than node1's. And no problem in the other direction to get 600 Mbit/s (with node1 CPU at 100%).