Have the same issue:
0%|          | 0/66 [00:00<?, ?it/s]
2%|▏         | 1/66 [05:53<6:23:05, 353.62s/it]
3%|▎         | 2/66 [06:11<4:29:46, 252.91s/it]
5%|▍         | 3/66 [06:28<3:11:06, 182.02s/it]
6%|▌         | 4/66 [06:41<2:15:39, 131.29s/it]
8%|▊         | 5/66 [06:43<1:34:15, 92.71s/it]
9%|▉         | 6/66 [06:46<1:05:41, 65.70s/it]
11%|█         | 7/66 [06:49<46:00, 46.79s/it]
12%|█▏        | 8/66 [06:51<32:26, 33.56s/it]
14%|█▎        | 9/66 [06:54<23:04, 24.30s/it]
15%|█▌        | 10/66 [06:57<16:37, 17.81s/it]
17%|█▋        | 11/66 [06:59<12:09, 13.27s/it]
18%|█▊        | 12/66 [07:02<09:04, 10.09s/it]
20%|█▉        | 13/66 [07:05<06:56,  7.86s/it]
21%|██        | 14/66 [07:07<05:27,  6.30s/it]
Cannot understand this behavior… sometimes it takes 5 minutes for a mini batch or just a couple of seconds.

my first epoch took me just 5 minutes.

94%|█████████▍| 62/66 [05:06<00:15, 3.96s/it]
95%|█████████▌| 63/66 [05:09<00:10, 3.56s/it]
97%|█████████▋| 64/66 [05:11<00:06, 3.29s/it]
98%|█████████▊| 65/66 [05:14<00:03, 3.11s/it]