I compiled the file with your options on my box without problems, but on my box it is a gcc 7.3.1.

What happens, when changing the offending lines from:

Qt Code: 
  1. const char sig[] = ...
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to
Qt Code: 
  1. const char* sig = ...
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Or do you see another way to change the code to work around the compiler issue - bside writing the SIGNAL/SLOT signatures twice ?

Uwe

PS1: your compiler flags differ from those you find in qwtbuild.pri. At least -fno-math-errno -funsafe-math-optimizations should have an impact on the performance, when using methods from cmath. Also you should use a higher optimization level.
PS2: do you see an effect on your CPU usage, when enabling QwtPlotCurve::FilterPointsAggressive ?