But let's be clear about the stated goal of his investigation, and what progress he's made towards that goal. And let's be clear that if we believe that Trump conspired with the Russians to interfere in the 2016 US elections, we cannot settle for evidence of other crimes, in lieu of evidence for the crime we set out to prove. Nor should we be entirely complacent about an investigation that turns up crimes other than the ones it actually set out to discover.
Okay, there are a few things which are clear on that front.
The first is that Mueller runs a very tight ship with no leaks, and we don't yet know what he knows.
The second is that most of the Americans thus far indicted have made a plea bargain and, as a result, ended up with much lighter sentencing than would be expected for such crimes otherwise. From this we can glean that they have provided something of value to Mueller, and from the information we know about them we can say that it's not unlikely that Mueller is not just seeking lenient sentencing on crimes he can prosecute, but is choosing not to prosecute some crimes that he could.
The third is that this is a very sensitive investigation going, as it is, after a sitting president. If Mueller is sensible - and it very much appears that he is - he will want to get all his ducks in a row before starting to pull the trigger on his top-level targets.
The fourth is that many experts, former investigators, former prosecutors, etc. have said that Mueller is moving at lightning speed, given the size, complexity, and sensitivity of the investigation.
Taking all of that into account, the fact that no Americans have yet been indicted on charges relating to conspiracy or treason has no significance whatsoever. None.