100 % cpu ! help

I noticed that the server was starting to lag and looked at how much it was consuming and saw that it only uses 1 CPU 100% at thszervergép is point.

CPU: Intel Core i7-6700K - 4 GHz - 4 core(s)
RAM: 64GB - DDR4
Hard Drive(s): 2x 256GB (SSD (NVMe) PCIe)
Bandwidth: Unmetered @ 1Gbps