The TES Invocations metric represents the number of Tesselation Evaluation Shader invocations. The Tesselation Evaluation Shader is invoked once per fixed function tessellator output point.
The purpose of a Tesselation Evaluation Shader is to calculate the vertex positions for subdivided points that are output by the fixed function tessellator. The best way to improve performance is to minimize the number of TES Invocations, which can be done by decreasing the amount of tessellation performed. You can do this by either decreasing the number Tesselation Control Shader Invocations or decreasing the tessellation factors in the Tesselation Control Shader.