+1 on the EtreCheck report. As there are ZERO viruses for Mac OS out there, therefore a virus is an impossibility. It is possible for you to have been tricked into installing some other type of malware though. The first place to start is with the EtreCheck report which will tell us what has been installed and NOT provide any access to any personal information. It will also provide us with clues such possible hardware issues that can be causing the performance loss such as a failing hard drive. It takes just a few minutes to download, install, run a report and then save it. Then attach the report in your reply to this message.