You're referring to the New Economic Policy [0].

For both, the purpose was not only to get foreign investment, but to improve internal economic efficiency, since collectivization and central planning were lethally inefficient [1][2].

[0] https://en.wikipedia.org/wiki/New_Economic_Policy

[1] https://en.wikipedia.org/wiki/Great_Leap_Forward

[2] https://en.wikipedia.org/wiki/Great_Chinese_Famine

Certainly, internal economic efficiency was also an important factor. Lenin rapidly realized that central planning is an incredibly difficult task and that allowing markets allows for more efficiency. However, Stalin disagreed as he wanted faster heavy industry development than markets would allow.

Indeed. Stalin effectively ended the NEP in 1929, eliminating private ownership of farmland and collectivizing agriculture under state control. By 1931, he had reimposed state control over all industry and commerce [0][1]. And thus came the Soviet famines of 1932-1933 [2][3][4].

[0] https://www.britannica.com/event/New-Economic-Policy-Soviet-...

[1] https://en.wikipedia.org/wiki/Collectivization_in_the_Soviet...

[2] https://en.wikipedia.org/wiki/Soviet_famine_of_1932%E2%80%93...

[3] https://en.wikipedia.org/wiki/Holodomor

[4] https://en.wikipedia.org/wiki/Kazakh_famine_of_1931%E2%80%93...

Yes, this is made worse by the fact that Lenin previously tried to do what Stalin did and found that access to reliable data was a major issue, but Stalin still went through with it, partly because he expected war and partly because he thought he could do it better. This wasn't something we should give the benefit to Stalin for but in retrospect WW2 may have taken a turn for the worse in the opposite case.