Performance improvements #815

Job	Run time
list-examples	7s
examples/machine-learning/example_born_fully_connected.jl	5m 55s
examples/machine-learning/example_extended_source.jl	5m 38s
examples/machine-learning/example_extended_source_cnn.jl	6m 4s
examples/scripts/extended_source_lsqr.jl	5m 28s
examples/scripts/fwi_example_2D.jl	5m 49s
examples/scripts/fwi_example_ADloss.jl	6m 4s
examples/scripts/fwi_example_NLopt.jl	5m 41s
examples/scripts/fwi_example_constraints.jl	6m 16s
examples/scripts/fwi_example_minConf.jl	5m 55s
examples/scripts/fwi_example_studentst.jl	7m 57s
examples/scripts/fwi_gauss_newton.jl	6m 24s
examples/scripts/illum.jl	6m 3s
examples/scripts/imaging_conditions.jl	5m 31s
examples/scripts/lsrtm_2D.jl	6m 55s
examples/scripts/modeling_basic_2D.jl	6m 17s
examples/scripts/modeling_basic_3D.jl	6m 2s
examples/scripts/modeling_basic_elastic.jl	5m 10s
examples/scripts/modeling_extended_source_2D.jl	5m 34s
examples/scripts/modeling_extended_source_3D.jl	13m 30s
examples/scripts/modeling_medical_2D.jl	5m 30s
examples/scripts/modeling_wavefields_2D.jl	5m 34s
examples/scripts/splsrtm_2D.jl	7m 31s
	2h 20m 55s

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