Performance improvements #819

Job	Run time
list-examples	4s
examples/machine-learning/example_born_fully_connected.jl	5m 59s
examples/machine-learning/example_extended_source.jl	5m 35s
examples/machine-learning/example_extended_source_cnn.jl	5m 48s
examples/scripts/extended_source_lsqr.jl	5m 29s
examples/scripts/fwi_example_2D.jl	6m 14s
examples/scripts/fwi_example_ADloss.jl	7m 12s
examples/scripts/fwi_example_NLopt.jl	5m 35s
examples/scripts/fwi_example_constraints.jl	6m 17s
examples/scripts/fwi_example_minConf.jl	6m 48s
examples/scripts/fwi_example_studentst.jl	7m 19s
examples/scripts/fwi_gauss_newton.jl	6m 36s
examples/scripts/illum.jl	6m 20s
examples/scripts/imaging_conditions.jl	5m 35s
examples/scripts/lsrtm_2D.jl	7m 27s
examples/scripts/modeling_basic_2D.jl	6m 11s
examples/scripts/modeling_basic_3D.jl	5m 51s
examples/scripts/modeling_basic_elastic.jl	5m 30s
examples/scripts/modeling_extended_source_2D.jl	5m 15s
examples/scripts/modeling_extended_source_3D.jl	5m 20s
examples/scripts/modeling_medical_2D.jl	6m 4s
examples/scripts/modeling_wavefields_2D.jl	5m 45s
examples/scripts/splsrtm_2D.jl	6m 48s
	2h 15m 2s

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