Cybercriminals today have turned so sophisticated that they come up with all kinds of advanced viruses and malware, many of which would bypass conventional security software. The sophisticated malware of today dont cause great, visible changes to happen in the impacted systems or networks. Still, there are some changes or warning signs that would let you know that your system could be malware-infected. These include a noticeable decrease in system speed, when your system takes longer to boot up or when browsing the internet or transferring files happens very slowly, you can start looking for malware presence in your system.”]
Source: https://hackercombat.com/12-warning-signs-that-help-identify-malware-infection/