That's not really a good argument. Taken to the extreme that'd be saying that JS engines should intentionally be slow in order to be "secure". JIT the code then inject a bunch of nop loops everywhere - you'd still be "preventing" meltdown, spectre, and rowhammer, but waste less power doing it.
JS engines are already intentionally slow. Look at all the hacks done to avoid random JS getting its hands on a precise timing source. Last time they straight up did away with SharedArrayBuffer.
This is all done to keep JIT around despite the obvious and massive security impact.
