Double check with -c that the issue has to do with licencing (e.g. <path-to-matlab>/matlab.exe -c <path-to-license-file>). If it opens quickly then Matlab is having some difficulty with LM_LICENSE_FILE.
Some have recommended removing LM_LICENSE_FILE (and using MLM_LICENSE_FILE) but I couldn't because of other software. However, one of those software tools had their own unique license variable as well, so I moved the pertinent license info to that one. Now Matlab loads quickly.
I suspect that because the other tool licences in LM_LICENSE_FILE were accessed over a network, and there were several network servers, this throws Matlab off somehow. Seems like a good solution is to minimize what goes into the shared LM_LICENSE_FILE variable.