Anyone experienced speed issues with Artemis Labs SA-1?

Don't get me wrong; I do like the Artemis, as mentioned above.  I hope you will be able to resolve the problem.  And certainly, the other drive systems have their specific pesky issues.

It often surprises me that belt drive aficionados are willing to put up with so much fuss, to keep their turntables running at correct speed. The whole idea of a belt is a pain in the ass waiting to happen.

If you used any motor with a speed adjustment function, then you would not have to worry about matching the motor to the turntable. You would just observe the initial speed error and use the adjuster to correct it. Have you considered adapting the eclipse motor system sold by SOTA to the Artemis? If you are happy with the Eagle/Road Runner, you will be equally happy with the eclipse, since the two emanate from the same inventor.

The Artemis is a very thoughtful design which uses two pulleys, one to drive the belt and the other to push the belt up against the circumference of the platter so that the belt contacts the platter very nearly on its full circumference.  This is to eliminate belt creep and along with the noncompliant belt, it probably works.  Have you looked closely at the passive pulley?  Is there any friction in it at all, when you turn it by hand?  Could it be hanging up on anything? Likewise, can you check for bearing noise in the motor pulley? If the pulleys are blameless, then I'd think about the motor itself.  I always like to eliminate the easy stuff first.

The issue could be the cell phone app.  It stinks.  Does the music have unstable pitch?  Many have reported "problems" with speed after measuring rpm with the cell phone app. Apparently, a measurement of 32 rpm is not outside of the margin of error of the app.