@Ramasurinen
OK, here’s some things that helped me.
Try turning NetPriority up in your Replication menu on your object that has Smooth Sync.
Try turning the SendRate down on SmoothSync. The default value of 30 a second is probably too high really (only sends if variables have changed though). I usually see around 15 sends a second as a good send rate. With a SendRate of 15 your InterpolationBackTime will be good at default but if you lower the SendRate to below 10, you’ll want to raise your InterpolationBackTime. Read the InterpolationBackTime mouse over details for more information.
Let me know how it goes.
If none of that helps, about how many objects with SmoothSync are you trying to sync?