What flickering do you have? I don't see any. As for startup time, I dunno, seems to open in less than half a second for me, though on a relatively high end laptop. On a 10 year old machine it might suffer.
And yes, at times it flickers and or images can tear.
I mean why would I? If all I need is viewing a couple of photos every now and then, cropping and rotating one or two and drawing some circles on them to highlight something in a screenshot and Windows already does that then why bother with Irfan other than habit and nostalgia.