At the start of Run 2 in 2015, the LHC delivered proton-proton
collisions at a center-ofmass energy of 13 TeV. During Run 2 (years
2015-2018) the LHC eventually reached a luminosity of 2.1 x 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