The bit to understand here is that when it is doing a complete scan, your system will experience frequent lagging, hanging and delayed access/response from your input/interactions with the system, because the CPU is Hijacked by Defender.
The second is the Full Scan feature which may be scanning all files, when the computer either wakes up from sleep or when it is connected to a network, or if it is scheduled to run daily. The two most common reason for it to be consuming high CPU usage is the real-time feature which is constantly scanning files, connections and other related applications in real-time, which is what it is supposed to be doing (Protect In Real Time). The service associated with this program is the Windows Defender Service. Antimalware Service Executable is the name of the process MsMpEng (MsMpEng.exe) used by the Windows Defender program.