XINS performances are regularly measured. An old article about XINS performance shows that XINS could be even faster than the competition.
The number of logs written per call is one of the important factor that can slow down or increase the performance of an API. You are advice to only logs necessary messages.
Since XINS 3.1, GZip compressed data are also accepted. If not already done, you are adviced to configure the HTTP Server or the Servlet container to return compressed data.
If the Servlet container runs in a HTTP Server, it's better to configure the compression on the HTTP Server.