Well, like any virus you'd probably only get one by downloading some sort of file from the internet. So, don't do that. It's actually rather difficult for a hacker to get into your Linux system just because you have your browser open, or whatnot.
A lot of Linux users do keep some sort of AV around, so that they can check a file before they post it publicly or send it to a friend who uses Windows. You can have one that does not constantly run in the background, and just scan the files that you need to. Heck, there are several free ones.
Which GUI for Mint are you using? I presume Cinnamon, since that's the most Windows-like and is usually recommended to new users. If you really want to cut down on background utilities and memory usage consider Xfce. It's designed to be very minimalist. (In fact, it does lack some of the customization features in which you seem to be interested.) But I believe the 18.3 version of Mint with Xfce is still in beta. You could just use 18.2 if you wanted- it's still supported until 2021.
All things considered, though, you're fine with Cinnamon.
Well, honestly, part of that is just the lightning speed of any new system. Linux will eventually slow down a bit, too. You do need to go in a clean out logfiles and whatnot every now and again. But in my experience it's never as bad as Windows.
I'm a big Linux believer and as I mentioned I eventually went Mac at least in part because OS X is a flavor of Linux, and I still have not defenestrated completely. Which, frankly, angers me. My MacBook is dual-booted with Windows 7 solely so that I can run Wilbur. I'm not exaggerating- it's the only thing I have ever installed on that partition other than the Windows OS itself. If you have any interest in obscure third-party software (e.g. Wilbur) you will eventually find something that you can only run by running Windows somehow, whether that is dual-boot, a VM, or Wine, or whatever. The Mac fanatics will claim that you can always find a Mac alternative, but that's a lie. Never trust a zealot. I went to the Apple store to get help setting up my dual-boot, since I had zero experience with Mac at that time, and you would not BELIEVE how much those geektards resisted setting up Bootcamp for me. It took a half hour of debate and eventually me getting VERY angry to break through to them. "No, the software I want to run does not exist in any form for Mac." "Have you really looked?" "It's very obscure freeware, and one dude maintains it." "There has to be another option." "Do you really want to part ways with your liver that badly? Because that's where we're headed." It was the Windows nanny attitude personified. I lost a bit of respect for Apple that day.
By that point I had gotten so used to doing whatever I wanted in Linux that the attitude grated especially harshly.