Bug report: addVariable() has quadratic time complexity on QCQP's

Thanks Michaja. It is a performance issue that is due to the extra overhead we need in order to link variables with quadratic constraints---it is indeed within the getVariable() call that most of the time is spent. We will fix this issue in a future release, but please note that for efficiency it would be far better to aggregate both the generation of variables and the insertion of constraints, as in the following code:

def create_qcqp(size):
    import xpress as xp
    p = xp.problem()
 
    x = [[xp.var(name=f"v{i}"), xp.var(name=f"w{i}")] for i in range(size)]
    p.addVariable(x)
    p.addConstraint(x[i][0]**2 + x[i][1]**2 <= 0 for i in range(size))

    2,000:     0.020
    4,000:     0.039
    8,000:     0.077
   16,000:     0.152
   32,000:     0.301
   64,000:     0.619
  128,000:     1.271
  256,000:     2.700
  512,000:     5.643
 1,024,000:    11.469