Perhaps they can, internally -- I know a couple of INFPs, and have gotten a rare glimpse under the hood at their minds working. Ye cats.
But to the external world, they don't give *any* indication of it.
As far as the relentless, I find the INFP resembles an INTP in their, well, *depth* of sounding things out, for something which is an INFP value. INTPs do this for logical definitions of systems, but again, only for particular topics of interest.
The INTJ doesn't dive as deep, formally, but tends to suck in everything like a Hoover. It was in the sense of *NEVER* stopping, that I meant relentless.