I’m guessing that with 1.5 seconds for 60k samples, you are using a GPU. This result comes from the parallelization power of GPUs, quite simply… They are better at processing data in parallel, and as long as you have enough “room” (memory, parallel threads, cores…), it will be faster if you give it more data, because less data will be left after one batch…
“Real life” comparison: if a sawmill has the capacity to saw 20 logs at the same time, but is only fed one at a time, it will take longer than giving it 20 at a time…