How long it took to train a ResNet50 image recognition model by year

How long it took to train a ResNet50 image recognition model by year:

2015: 25 days
2017: 8 hours
2018: 7.1 minutes
2019: 1.28 minutes
2020: 0.47 seconds

Sources:
- https://blogs.nvidia.com/blog/2019/07/10/mlperf-ai-performance-records/
- https://mlperf.org/training-results-0-5
- https://mlperf.org/training-results-0-6
- https://mlperf.org/training-results-0-7
- https://cloud.google.com/blog/products/ai-machine-learning/google-breaks-ai-performance-records-in-mlperf-with-worlds-fastest-training-supercomputer