Is the phishing filter necessary? Websites load very slowly when the
phishing filter is turned on. If it's important for security I'll turn it
back on, otherwise, it is okay to leave it off?
Necessary? No.
It alerts you to web sites and E-mail messages that may not be what they
purport to be and may steal your private info if you enter it. If you think
you are astute enough to be able to recognize such by yourself, and you
don't need that help, you may certainly turn it off.
Personally, I think I'm pretty good at recognizing phishing without
Microsoft's help, but I keep it on anyway. I want all the extra protection I
can get, because I know that however good I am, I'm not perfect.
Your choice.