That's really not the case. Once you know of an attack of this sort it's pretty easy to test everyone's chips for it. Bascially every Intel chip since Nehalem is vulnerable to Meltdown, as are IBM's POWER chips, as is one out-of-order ARM core but not any of the others. And we know that AMD chips are safe from that vulnerability.

Whether you run security checks in sequence or in parallel with a data access is pretty fundamental to the design of a core. Doing the later is has performance advantages but it's really hard to verify that won't be any architectural leaks as a result, let alone these micro-architectural leaks that nobody was thinking about.