__lll_lock_wait allowing only one thread to work

I am not denying that there is bottleneck with the mutexes. I am trying to understand what causes this bottleneck and what are the situations in which multiple threads would acquire a global lock.

I don't think you broke anything by switching to single-threaded. Xpress parses /proc/meminfo. If this happens from many different threads at the same time, then it is not out of question that you get an EMFILE (24, "too many open files") error. It seems that now many threads attempt to access this file simultaneously and hit this limit. Would it be an option to increase the limit on open files to work around this problem?

Alternatively, could you set the TREEMEMORYLIMIT control (see https://www.fico.com/fico-xpress-optimization/docs/latest/solver/optimizer/HTML/TREEMEMORYLIMIT.html)? That should cut back on the reading of /proc/meminfo.