I use a few apps from this developer and I have never had any problems with them. KnockKnock doesn’t slow anything down, and isn’t an antivirus app per se (though it integrates data from the separate VirusTotal, which is owned by Google). From the FAQ:
Q: KnockKnock found many applications, should I be worried? **
A: No. KnockKnock simply enumerates items that are automatically started; either during startup, during login, or during another application’s launch (e.g. browser extensions). Although signed-Apple items are filtered out by default, many legitimate 3rd-party items will likely be shown. Of course, the goal is that KnockKnock will also display any persistently installed malware.
Q: Ok, so how do I determine if something is malware?
A: By design KnockKnock itself doesn’t try to determine if something is malware or not. However, since VirusTotal is fully integrated into KnockKnock, known malware will be detected (and highlighted in red). The remaining items that are not flagged can be manually examined. Perhaps google the hash of the file, run strings on it, or if you are really concerned about a specific item, email me at firstname.lastname@example.org and attach the file