Uibutton with png makes uiscrollview linger

I have a UIScrollView with 15 UIButtons (create with storyboard)

When I zoom in or scroll it.

Scrolling trail:

Running Time Self Symbol Name 434.0ms 100.0% 0,0 Main Thread 0x61091 434.0ms 100.0% 0,0 start 434.0ms 100.0% 0,0 main 434.0ms 100.0% 0,0 top_level_code 434.0ms 100.0% 0,0 UIApplicationMain 434.0ms 100.0% 0,0 GSEventRun 434.0ms 100.0% 0,0 GSEventRunModal 434.0ms 100.0% 0,0 CFRunLoopRunInMode 434.0ms 100.0% 0,0 CFRunLoopRunSpecific 434.0ms 100.0% 0,0 __CFRunLoopRun 417.0ms 96.0% 0,0 __CFRunLoopDoObservers 417.0ms 96.0% 0,0 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ 415.0ms 95.6% 0,0 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) 413.0ms 95.1% 0,0 CA::Transaction::commit() 413.0ms 95.1% 0,0 CA::Context::commit_transaction(CA::Transaction*) 406.0ms 93.5% 0,0 CA::Layer::prepare_commit(CA::Transaction*) 406.0ms 93.5% 0,0 CA:: ::prepare_image(CGImage*, CGColorSpace*, unsigned int, double) 406.0ms 93.5% 0,0 CA::Render::copy_image(CGImage*, CGColorSpace*, unsigned int, double) 406.0ms 93.5% 0,0 CA::Render::create_image(CGImage*, CGColorSpace*, unsigned int) 406.0ms 93.5% 0,0 CGImageProviderCopyImageBlockSetWithOptions 406.0ms 93.5% 0,0 ImageProviderCopyImageBlockSetCallback 406.0ms 93.5% 0,0 copyImageBlockSetPNG 331.0ms 76.2% 0,0 _cg_png_read_row 207.0ms 47.6% 1,0 png_read_IDAT_data 203.0ms 46.7% 12,0 inflate 177.0ms 40.7% 0,0 <Unknown Address> 168.0ms 38.7% 166,0 0x257f24e 2.0ms 0.4% 0,0 i386_astintr 2.0ms 0.4% 2,0 ast_taken 3.0ms 0.6% 3,0 0x257f0cb 2.0ms 0.4% 2,0 0x257ef8b 2.0ms 0.4% 2,0 0x257efcb 1.0ms 0.2% 1,0 0x257f1f0 1.0ms 0.2% 1,0 0x257effb 10.0ms 2.3% 0,0 adler32 3.0ms 0.6% 0,0 0x257c67c 1.0ms 0.2% 1,0 0x257eee0 2.0ms 0.4% 0,0 png_crc_read 1.0ms 0.2% 0,0 <Unknown Address> 1.0ms 0.2% 1,0 0x257eee0 68.0ms 15.6% 0,0 png_read_filter_row 47.0ms 10.8% 7,0 png_do_read_transformations 7.0ms 1.6% 1,0 png_combine_row 1.0ms 0.2% 0,0 png_read_finish_IDAT 1.0ms 0.2% 1,0 _platform_memmove$VARIANT$sse42 69.0ms 15.8% 18,0 _platform_memmove$VARIANT$sse42 50.0ms 11.5% 10,0 user_trap 37.0ms 8.5% 3,0 vm_fault_internal 2.0ms 0.4% 2,0 kernel_debug 1.0ms 0.2% 0,0 <Unknown Address> 1.0ms 0.2% 1,0 thread_exception_return 2.0ms 0.4% 0,0 _cg_png_read_info 1.0ms 0.2% 0,0 ImageIO_make_read_only 1.0ms 0.2% 0,0 _cg_png_destroy_read_struct 1.0ms 0.2% 0,0 CGImageRead_mapData 1.0ms 0.2% 0,0 <Unknown Address> 1.0ms 0.2% 1,0 png_read_IDAT_data 7.0ms 1.6% 0,0 CA::Layer::layout_and_display_if_needed(CA::Transaction*) 1.0ms 0.2% 1,0 objc_autoreleasePoolPush 1.0ms 0.2% 0,0 (anonymous namespace)::AutoreleasePoolPage::pop(void*) 2.0ms 0.4% 0,0 _afterCACommitHandler 16.0ms 3.6% 0,0 __CFRunLoopDoSources0 1.0ms 0.2% 0,0 __CFRunLoopDoTimer

Is there a way to avoid this lag? why is uiimage rendered every time? isn't it possible to do it in the background?

PS: the whole button is visible in the view.

Many thanks for your help