If you're connected to the internet, a firewall is really needed no matter what.
Once any malicious program leaks your sensitive data or receives malicious payload despite an AV watching, it's of
no use inspecting the graphs of old/already done connections in GlassWire.
Why don't you post your system specs
here for us to know how well will it do?
People here can recommend you a light FW. You can try out
Tinywall (light, tight rules) or
Zonealarm FW (more customizable, interactive, bigger one), though you can get to know more over
here. These won't have much effect on speed, try out.
And yes, major part of protection is your decision. Allow only know programs to connect. Take recommendations that FWs give when you're not sure (eg. Zonealarm does).