The original CDJ 900 also has terrible master tempo. The original CDJ 2000's got a software fix that made it tolerable but just barely. I guess the 850 and 900 models just doesn't have enough hardware power to support a decent master tempo algorithm :(
Sadly , the only advice is to never use the master tempo function, learn to work without it. On the upside, the CDJ 2000 and 900 Nexus master tempo is absolutely awesome.