I am using the latest UAD v6.5.2 with SONAR X2a x64 on my Win7 (SP1) x64 DAW with no issues whatsoever as well. If you are using all x64 (UAD plug-ins, OS & DAW app) you need to make sure that SONAR is not scanning any folders with 32bit (x86) UAD plug-ins. You need to be using x64 UAD plug-ins exclusively. Having SONAR scan
both x86 & x64 plug-ins will cause severe performance issues. SONAR can only use one or the other and that should be the x64 plug-ins. Ditch the UAD x86 bit-bridged plug-ins and you should be good to go. Either delete them or move them to another folder and don't have SONAR x64 scan them
ever. After doing this it would be best if you go into the SONAR plug-in manager and do a "reset" and fresh rescan and you should be all set.
I would also recommend installing and use the "optional" mono (m) UAD plug-ins. For DAW apps like SONAR it is a must, if you want to enjoy the reduced DSP usage you get with using the mono (m) versions. Otherwise, you will always get the full DSP load when using the standard UAD plug-ins on mono tracks/sources.
Cheers,
Billy Buck