If there really is a slowdown, it's not significant enough for either Jon or I to have noticed it.
The slowdown warning is, I believe, related to
Mojave (MacOS 10.14) which I do not believe either you or Jon are running.
There appears to be some confusion about that; I saw similar language on AdBlock's website.
The
slowdown warning is actually a function of Safari 12, regardless of OS. (You're correct about my not running Mojave.)
One thing I am pretty sure you will not like in Mojave is the necessity of going to System Preferences > Security & Privacy after installing or updating many apps to specifically grant that app access to various system functions. My wife calls this Apple's anti-FBI strategy. No it cannot be turned off.
I don't hate ALL security; I'll have no trouble dealing with it on a once/app basis.