I've been exploring various solutions to avoid image decompression on the main thread in order to speed up rendering images. The developer community has previously shared various ways of preloading or inflating images on the background before rendering them, and most of the examples out there still work today with varying degrees of speed. See cocoanetics dot com/2011/10/avoiding-image-decompression-sickness/, gist dot github dot com/steipete/1144242, and github dot com/Alamofire/AlamofireImage/blob/3e8edbeb75227f8542aa87f90240cf0424d6362f/Source/UIImage%2BAlamofireImage.swift#L113.
What's unfortunate is that none of the decompression techniques are documented, so the community is basing these assertions off of trial and error and hoping they don't regress between iOS versions. And what's more, is that some solutions are potentially ripe with error, likely missing critical alpha data, scale, orientation, or other bitmap info that might produce artifacts in the final rendered image.
In our situation, we are already served derivatives of images from the server that are exactly the size that they will be rendered. But we have found through profiling in instruments that there is a huge performance win by pre-inflating the images on a background thread. So we are left with deciding what approach to take for inflation.
In order to validate the inflation is working, there are a few ways to observe the decompression happen during runtime.
Here is a typical backtrace of seeing PNG decompression:
	15 CoreFoundation 209.0	CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION
	14 QuartzCore 180.0	CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*)
	13 QuartzCore 180.0	CA::Transaction::commit()
	12 QuartzCore 180.0	CA::Context::commit_transaction(CA::Transaction*, double)
	11 QuartzCore 180.0	CA::Layer::prepare_commit(CA::Transaction*)
	10 QuartzCore 180.0	CA::Render::prepare_image(CGImage*, CGColorSpace*, unsigned int, double)
	 9 QuartzCore 180.0	CA::Render::copy_image(CGImage*, CGColorSpace*, unsigned int, double, double)
	 8 ImageIO 180.0	IIOImageProviderInfo::CopyImageBlockSetWithOptions(void*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*)
	 7 ImageIO 180.0	IIO_Reader::CopyImageBlockSetProc(void*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*)
	 6 ImageIO 180.0	PNGReadPlugin::copyImageBlockSet(InfoRec*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*)
	 5 ImageIO 180.0	PNGReadPlugin::DecodeUncomposedFrames(IIOImageRead*, GlobalPNGInfo*, ReadPluginData const&, PNGPluginData const&, std::1::vector<IIODecodeFrameParams, std::1::allocator<IIODecodeFrameParams> >&)
	 4 ImageIO 180.0	PNGReadPlugin::DecodeFrameStandard(IIOImageReadSession*, ReadPluginData const&, PNGPluginData const&, IIODecodeFrameParams&)
	 3 ImageIO 180.0	_cg_png_read_row
	 2 ImageIO 159.0	png_read_IDAT_data
For any readers, in lldb, calling:
(lldb) br s -n _ZN13PNGReadPlugin22DecodeUncomposedFramesEP12IIOImageReadP13GlobalPNGInfoRK14ReadPluginDataRK13PNGPluginDataRNSt3__16vectorI20IIODecodeFrameParamsNSA_9allocatorISC_EEEE
Breakpoint 1: where = ImageIO`PNGReadPlugin::DecodeUncomposedFrames(IIOImageRead*, GlobalPNGInfo*, ReadPluginData const&, PNGPluginData const&, std::1::vector<IIODecodeFrameParams, std::1::allocator<IIODecodeFrameParams> >&), address = 0x00007fff2615e4d4
This will set a symbolic breakpoint that will TRAP on PNGReadPlugin::DecodeUncomposedFrames
Here is a typical backtrace of seeing JPEG decompression:
	16 CoreFoundation 13.0	CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION
	15 QuartzCore 13.0	CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*)
	14 QuartzCore 12.0	CA::Transaction::commit()
	13 QuartzCore 12.0	CA::Context::commit_transaction(CA::Transaction*, double)
	12 QuartzCore 12.0	CA::Layer::prepare_commit(CA::Transaction*)
	11 QuartzCore 12.0	CA::Render::prepare_image(CGImage*, CGColorSpace*, unsigned int, double)
	10 QuartzCore 12.0	CA::Render::copy_image(CGImage*, CGColorSpace*, unsigned int, double, double)
	 9 ImageIO 12.0	IIOImageProviderInfo::CopyImageBlockSetWithOptions(void*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*)
	 8 ImageIO 12.0	IIO_Reader::CopyImageBlockSetProc(void*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*)
	 7 ImageIO 12.0	AppleJPEGReadPlugin::copyImageBlockSet(InfoRec*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*)
	 6 AppleJPEG 11.0	applejpeg_decode_image_all
	 5 AppleJPEG 10.0	aj_decode_all_mt
	 4 AppleJPEG 10.0	aj_decode_all
	 3 AppleJPEG 6.0	fill_coeff_buffer
	 2 AppleJPEG 6.0	aj_mcu_decode
	 1 AppleJPEG 5.0	aj_block_decode
	 0 AppleJPEG 2.0	aj_huffman_decode_ac_s1
For any readers, in lldb, calling:
(lldb) br s -n _ZN19AppleJPEGReadPlugin17copyImageBlockSetEP7InfoRecP15CGImageProvider6CGRect6CGSizePK14__CFDictionary
Breakpoint 1: where = ImageIO`AppleJPEGReadPlugin::copyImageBlockSet(InfoRec*, CGImageProvider*, CGRect, CGSize, __CFDictionary const*), address = 0x00007fff262a53b8
This will set a symbolic breakpoint that will TRAP on AppleJPEGReadPlugin::copyImageBlockSet
marksands2.