Not that much. Each generation there was maybe 7% perf improvement so in 6 years: 1.07^6=1.50. Improvement was mostly with adding more cores.
This ~1.5x in single thread performance improvement matches with stats from geekbench.com benchmark:
my macbook pro 2012 cpu:
i7-3615QM => 3093 single thread [1]
equivalent current gen cpu with same TDP (45W) and similar clock (2.3GHz):
i7-8750H => 4617 single thread [2]
Before or after meltdown, spectre, l1tf, foreshadow, zombieload, ridl, and fallout patches? I would honestly guess that these new round have cost a huge amount of progress. I certainly won't be applying the latest three; I spent too much on a nice machine to have it perform as though it were four years old.
How much has x86 performance increased over the past six years?