High usage of jenkins cpu without work

The problem is this: I noticed that the CPU usage is 100% when starting the build. Assembly time is approx. 2 hours. If the build finishes, the Jenkins usage is still 80-85%. After that I restart the jenkins service. CPU utilization was 5%. I started the same build and it lasted 38 minutes. When starting the build, the utilization was 100%, but for this task it is ok if the duration is 38 minutes. This morning at 7 o'clock the CPU consumption is 80%. I am not starting the build at this time. If I build the project, its duration is already 2h. This project has 3 tasks. The duration of the rest is 20-25 minutes.

Maybe someone can find the problem. Tell me if you need any information.

Jenkins version: 1.590 Server: Windows 7, 32bit