I reexamined and confirmed that pet spellspeed are also contributed by master's spellspeed. It seems it works in the same manner as 2.16 (tooltip is unmodified but actual cast and recast are modified). I struck through some of my previous posts that was wrong.

Fortunately, my embarrassing struggle have left me the data collection and the neat way for analyzing. I will use them to reanalyze pet crit and spellspeed. (I updated pet crit information above).