Solved by Chame_call in post #14 
              
              
                Setting environment variable LD_PRELOAD with the aim of loading jemalloc instead of default CPU allocator solved the problem. 
My launch is as follows: 
LD_PRELOAD=./libjemalloc.so.1 python3 app.py. 
Related links: 

Related problem
Decreasing RAM Usage by 40% Using jemalloc with Python & Celery