I'll say some people are very easy to clear because there is direct evidence that shows they were not near the scene of a crime. Other people, who may be just as innocent, are not easily officially cleared because there is not direct evidence of their location.
Imagine a crime takes place in Moscow, Idaho. They think my client did it. My client was on a plane from Boise to LA during that time. LE can quickly confirm checking: plane ticket, if my client boarded, security camera at the airport in Boise, security camera at the airport in LA. They can conclusively clear him based on direct evidence pretty quickly.
Now what if instead my client says he was at his mom's house in Moscow? How do we clear him then? If there's no security cameras showing him coming (and not leaving), you are left looking for witnesses, subpoenaing phone and car GPS, canvasing the neighborhood for security cameras. It may take a while to confirm yes - client WAS home and could not have committed this crime. Or maybe there's no direct evidence either way and you are left tabling someone until you can figure it out.
So IMO "clearing" or "not clearing" someone can be a process. People easily cleared usually means there's some good evidence they were elsewhere. Others may just take longer to decide.