Depends on your risk. If you do questionable things, and especially if you don't make OS partition backups on a fairly regular basis so you can nuke things from orbit, then you need a stronger front line of protection, that will slow down your system. If you do online banking or professional (sensitive) data it is also higher risk, as is letting anyone else use your system while unattended.
If you have a newer system and modern SSDs, you may not even notice the slowdown, though newer software, bigger multimedia with higher compression, more bloated websites, etc, it's a continual journey to get the performance you need, when you need it. Point is you will need to upgrade again sooner with each upgrade cycle.
Remember that you can just uninstall anything that you find slow you down too much, or another case that turns me off is when they are too intrusive. I once installed something that wouldn't even let me run a dos batch file which I thought was ludicrus, but apparently THEY decided for everyone, what should be allowed. Sometimes you can go through the security software settings and deactivate features. Sometimes you can't or it's a PITA to find how they buried the setting and decided not to use enough text to make it clear what each setting was, because keeping users ignorant with lots of artistically pleasing empty white space is apparently better than information. [/rant]
95% of it is a secure browser, not opening unknown email attachments, and having a router between the system and modem. You can go further and even use a block-all-except-allowed apps through firewalled specific ports, but again it is about your own risk.
Windows Defender is good for what it is. Some people need better and some don't.
