Slowing add-on changes
It's known that Myers add-ons that slow him down are considered weak. If tombostone is arguable, since it's "a free mori" perk (though, comparing to a piece, it slows you, takes longer to charge, and the fact that it doesn't end t3 is pointless since how long mori is and nobody gonna stick around anyway), vanity mirror goes straightly to the garbage bin because it removes all benefits of t1, locks you out from t3 and gives pretty weak 16m aura reading.
Now, there was a precedent not so long ago based on movement speed of killers affecting their TR, in order to make everything consistent. So, I think that rule should be applied to all add-ons that permanently modify killer speeds.
Myers: Vanity Mirror - slowdown changes speed by 0.2, reduces TR by 8m;
Myers: Judith's Tombstone - slowdown changes speed by 0.2, reduces TR by 8m.
Nurse: Matchbox - no change because her TR still stays bigger than her walking speed suggests.
I don't remember any other add-ons that permanently change killer speed :/