No. The iPhone has not really improved since iP4 -- Apple has stopped innovating. I will likely upgrade from iP5 to a WP, namely Lumia 1020. Laggy and poorly-made Androids are not an option. It is either an iP (boring but safe) or a Lumia (innovative but MS-bound).
Funny,,,,,, as a former iphone junkie (owned every model) I sold my ip5 and moved to the Note 2 and now have the GS4,,,,, I have not experienced this laggy thing you mentioned at all with my Androids,,,, I have not found them poorly made either,,,,,, I would agree that the iphone 5 feels more premium and better made,,,,, but that does not mean the Note 2 or GS4 is poorly made.
Anyhow,,,,,,, I'm hoping for more than a tiny upgrade with the iPhone 5s or 6,,,,,,with a bigger screen and a nice upgrade and I might come back to iOS.
Cheers
Davyo