The reason why it uses so many resources is that of its portability. Most anti-viruses you need to install and keep constantly running. However, while using ZHP, you can just run it whenever you chose so. Generally, Anti-virus software uses a decent amount of resources if your computer is weaker concerning specifications. The scan time is dependent on many factors, including your CPU and HDD/SSD speeds, although while using SSD it shouldn't cause any issues. And in general, during an active scan, your computer will be slower regardless of the software, because it needs resources to perform active checks. Have you tried other anti-virus solutions? Do they also slow down your computer as much?