Though I’m curious, how much time does it take to fetch random indices from ds_all_utts in average ? Usually even for big datasets it stays below 1ms.

I ran this:
import time

for idx in range(0, len(ds_all_utts), 100000):
    t0 = time.time()
    row = ds_all_utts[idx]
    print(f"fetching idx {idx} of {len(ds_all_utts)} took {time.time()-t0} seconds")

…and found that it takes longer for higher indexes:
fetching idx 0 of 2732013 took 0.0025310516357421875 seconds
fetching idx 100000 of 2732013 took 0.018671512603759766 seconds
fetching idx 200000 of 2732013 took 0.027704238891601562 seconds
fetching idx 300000 of 2732013 took 0.03670859336853027 seconds
fetching idx 400000 of 2732013 took 0.04753923416137695 seconds
fetching idx 500000 of 2732013 took 0.06029653549194336 seconds
fetching idx 600000 of 2732013 took 0.07588982582092285 seconds
fetching idx 700000 of 2732013 took 0.08530783653259277 seconds
fetching idx 800000 of 2732013 took 0.09908699989318848 seconds
fetching idx 900000 of 2732013 took 0.10945987701416016 seconds
fetching idx 1000000 of 2732013 took 0.11123108863830566 seconds
fetching idx 1100000 of 2732013 took 0.10822105407714844 seconds
fetching idx 1200000 of 2732013 took 0.11907958984375 seconds
fetching idx 1300000 of 2732013 took 0.13410329818725586 seconds
fetching idx 1400000 of 2732013 took 0.15185165405273438 seconds
fetching idx 1500000 of 2732013 took 0.20782732963562012 seconds
fetching idx 1600000 of 2732013 took 0.18477439880371094 seconds
fetching idx 1700000 of 2732013 took 0.3052663803100586 seconds
fetching idx 1800000 of 2732013 took 0.21129512786865234 seconds
fetching idx 1900000 of 2732013 took 0.22873163223266602 seconds
fetching idx 2000000 of 2732013 took 0.24343037605285645 seconds
fetching idx 2100000 of 2732013 took 0.25824809074401855 seconds
fetching idx 2200000 of 2732013 took 0.34645700454711914 seconds
fetching idx 2300000 of 2732013 took 0.29653358459472656 seconds
fetching idx 2400000 of 2732013 took 0.3089408874511719 seconds
fetching idx 2500000 of 2732013 took 0.32140231132507324 seconds
fetching idx 2600000 of 2732013 took 0.3361659049987793 seconds
fetching idx 2700000 of 2732013 took 0.41907739639282227 seconds