The strength in this method is that it has the littlest possible surface area for upstream bugs to compromise your final entropy. Because, in the applied world, upstream bugs in "secure" system RNGs have been the cause of stolen crypto and other critical security compromises on numerous occasions.

And, I agree that if the system is compromised to the level that the attacker can control the output of the timer, it's probably compromised to the level that the attacker can just read your generated entropy straight from memory.

The point here is not to be fast, it's to be protected against implementation bugs on systems that weren't designed by security professionals.