You're measuring a cached compile in the subsequent runs. The deps.compile probably did some native compilation in the dep folder directly rather in _build.
rm -rf _build/ deps/ && mix deps.get && time MIX_OS_DEPS_COMPILE_PARTITION_COUNT=1 mix deps.compile
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Executed in 37.75 secs fish external
usr time 103.65 secs 32.00 micros 103.65 secs
sys time 20.14 secs 999.00 micros 20.14 secs
rm -rf _build/ deps/ && mix deps.get && time MIX_OS_DEPS_COMPILE_PARTITION_COUNT=5 mix deps.compile
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Executed in 16.71 secs fish external
usr time 2.39 secs 0.05 millis 2.39 secs
sys time 0.87 secs 1.01 millis 0.87 secs
rm -rf _build/ deps/ && mix deps.get && time MIX_OS_DEPS_COMPILE_PARTITION_COUNT=10 mix deps.compile
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Executed in 17.19 secs fish external
usr time 2.41 secs 1.09 millis 2.40 secs
sys time 0.89 secs 0.04 millis 0.89 secs mise use elixir@1.19-otp-26 erlang@26
rm -rf _build/ deps/ && mix deps.get && time MIX_OS_DEPS_COMPILE_PARTITION_COUNT=1 mix deps.compile
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Executed in 97.93 secs fish external
usr time 149.37 secs 1.45 millis 149.37 secs
sys time 28.94 secs 1.11 millis 28.94 secs
rm -rf _build/ deps/ && mix deps.get && time MIX_OS_DEPS_COMPILE_PARTITION_COUNT=5 mix deps.compile
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Executed in 42.19 secs fish external
usr time 2.48 secs 0.77 millis 2.48 secs
sys time 0.91 secs 1.21 millis 0.91 secs