The most common way to get malware on a Mac is to install a piece of dodgy software. If you've done that, you'll likely see symptoms that are worse than slowness, though. Probably the first thing one can do is to make sure Gatekeeper is set to its default. In System Preferences, choose Security & Privacy, hit the General tab, then check to make sure that "Mac App Store and identified developers" is chosen. That will cause the system to produce a warning if an app doesn't come from a registered developer. Choosing "Mac App Store" is even more strict, but the one you want to avoid is "Anywhere." "Mac App Store and identified developers" isn't perfect, but developers know that if they don't follow the rules, their accounts will be revoked. Macs have built in anti-malware protection called XProtect that runs silently in the background. I don't think that it's possible to shut it off, so you're likely covered there. It's going to cover online threats rather than dodgy software installations, though. As an example, it shuts down the ability to use old versions of Java and Flash. It's actually even shut down the current version of Java a couple of times. Still, I think you're covered by running Malwarebytes now and then. A full-time anti-malware app running in the background may make things slower than they already are. Speaking of Java and Flash, you probably don't need Java at all. Almost all Mac exploits that didn't require users to install software have involved Java. Having said that, I think Java's security record has improved over the past couple of years. Flash sucks security-wise too, but I don't think that any of the serious Flash exploits have ever turned into attacks on the Mac. Make sure it's always up to date. And always get it from Adobe's site. One of my friends installed a fake Flash. It took over Safari and installed a fake Bing, and it caused her to get popups telling her to call a fake Microsoft for support. It also dropped three apps into her Applications folder, which thankfully, she didn't open. I've decided that I can get by without Flash being globally installed. For the few times I run into a page that has Flash content that interests me, I fire that page up in Google Chrome. Chrome has Flash sandboxed within the browser. One common cause of Mac slowness is a hard drive (or SSD) that's too full. How does that look? Another cause is too many items on the desktop. Figure that each icon loads a full preview of the document it represents. If you have a tendency to want to put a lot of stuff on the desktop, it's better to put a folder there and stash these items in it. You might gain some speed by doing a safe boot. Doing this will clean out some stuff (caches and such). To do a safe boot, hold down the shift key during startup. A safe boot takes significantly longer than a normal startup, so don't be alarmed. Once you've booted into in safe mode, simply restart normally. This restart will take a hair longer than most boots because nothing's cached. You don't actually have to do anything while you're in safe mode. Simply doing the safe boot will take care of the cleanup tasks. You can also check your login items to make sure you aren't running any unnecessary items at startup. To do this, go to System Preferences, hit the Users & Groups icon, then hit the Login Items tab. To remove a login item, simply select it, then hit the "minus" button below the list.