  At the start of Run 2 in 2015, the LHC delivered proton-proton collisions at
a center-of-mass energy of 13 TeV. During Run 2 (years 2015-2018) the LHC
eventually reached a luminosity of 2.1 $\times$ 10$^{34}$ cm$^{-2}$ s$^{-1}$,
almost three times that reached during Run 1 (2009-2013) and a factor of two
larger than the LHC design value, leading to events with up to a mean of about
50 simultaneous inelastic proton-proton collisions per bunch crossing (pileup).
The CMS Level-1 trigger was upgraded prior to 2016 to improve the selection of
physics events in the challenging conditions posed by the second run of the
LHC. This paper describes the performance of the CMS Level-1 trigger upgrade
during the data taking period of 2016-2018. The upgraded trigger implements
pattern recognition and boosted decision tree regression techniques for muon
reconstruction, includes pileup subtraction for jets and energy sums, and
incorporates pileup-dependent isolation requirements for electrons and tau
leptons. In addition, the new trigger calculates high-level quantities such as
the invariant mass of pairs of reconstructed particles. The upgrade reduces the
trigger rate from background processes and improves the trigger efficiency for
a wide variety of physics signals.
