McShield is the process that loads our scanning engine. It scans files as they are accessed in real time. This will mean it uses CPU.
Other scan components, like on-demand scans and email scans or script scans, communicate with McShield to request an object be scanned. This means McShield is the one using CPU for those scan requests too.
So, if you see McShield using CPU, you can be confident it is acting in response to a scan request. Lots of CPU activity should be interpreted as lots of scan requests occurring.
Therefore to reduce McShield CPU usage, you need to reduce the number of scan requests that are occurring...
Your first step should be to identify what is being scanned. Next, determine if any of those items being scanned can _not_ be scanned.
It is also possible you could be experiencing a code problem, where McShield is "spinning out of control" and using all the available CPU on one of your CPU cores. If you suspect that is the case, make sure you have the latest available updates for the product.