In case that helps: Repro on TitanV with Pytorch 1.3.0.dev20190910 (nightly) with CUDA10.0 and NVIDIA driver 418.56:
=====Test1=====
Discriminator backward :  0.0056169ms
Generator backward :  0.0103526ms
Hessian vector product :  0.0136104ms
=====Test2=====
Discriminator backward :  0.0149076ms
Generator backward :  0.0242238ms
Hessian vector product :  1.0203860ms

(just added torch.cuda.synchronize() before starting and stopping the timers, no warmup etc.)