In ghci, using the arithmoi package:
Math.NumberTheory.Powers.General> :set +s
Math.NumberTheory.Powers.General> integerRoot 786 ((10^32)^786)
100000000000000000000000000000000
(0.04 secs, 227,064 bytes)
Math.NumberTheory.Powers.General> integerRoot 787 ((10^32)^787)
After five minutes, it still hasn't responded. Why is it taking so long?
(From some ad-hoc testing, it appears to be slow for all choices larger than 787 and fast for all choices smaller.)