What is exactly a uniform server in a galera cluster setup? In many manuals about performance tuning and avoid Galera's MySQL cluster limitations they write the below suggestion. 


  It is possible for a slow/overloaded master node to affect performance
  of the Galera Replication cluster, therefore it is recommended to have
  uniform servers across the cluster.


I cannot find any information about what a uniform server is? Has anybody an idea?
The word "uniform" is being used to describe the fact that each server should have similar or identical resources, in terms of cpu, memory, disk (seek time, I/O bandwidth), etc.

Since a single node can slow down the entire cluster, the slowest node determines the overall capacity of the cluster... so using servers whose performance capabilities are not "uniform" (closely comparable) will mean that resources on the more powerful machines will go underutilized, while the less powerful machines slow down the cluster.