IDL 8.7.3 was released recently1. The new features are:

1. Using the Intel Math Kernel Library (MKL) for matrix operations now. My experience has been that MKL is a highly optimized implementation of BLAS, a library of low-level matrix and vector operations. Matrix operations, such as the # and ##, should be faster now, but I don’t have 8.7.3 to test on, yet.
2. New BLASS_GEMM matrix multiplication routine with many options for controlling the operation. This is a consequence of adding the MKL.
3. New SOBOLSEQUENCE routine to generate numbers from the Sobel sequence.

UPDATE 2/25/2020: I have IDL 8.7.3 now and did some simple testing of the performance between IDL 8.7.3 and IDL 8.6 for # and ##. Mean time for multiplying two 50,000 by 50,000 element matrices went from 76.942 sec in IDL 8.6 down to 3.400 sec in IDL 8.7.3.

1. I have no idea exactly when or how to find that out. ?