safe_lock is nthreads() > 1 by default

src/ProgressMeter.jl

@@ -81,7 +81,7 @@ Base.@kwdef mutable struct ProgressCore
     numprintedvalues::Int       = 0             # num values printed below progress in last iteration
     prev_update_count::Int      = 1             # counter at last update
     printed::Bool               = false         # true if we have issued at least one status update
-    safe_lock::Bool             = true          # set to false for non-threaded tight loops
+    safe_lock::Bool             = Threads.nthreads() > 1 # set to false for non-threaded tight loops
     tinit::Float64              = time()        # time meter was initialized
     tlast::Float64              = time()        # time of last update
     tsecond::Float64            = time()        # ignore the first loop given usually uncharacteristically slow