The only online retailer I have a problem with in regards to delivery is 7 Day Shop, who seem to take their name as a starting point for delivery times. I've never had anything from them in less than 7 days (similar to More Than insurance, who's quotes are always More Than just about everyone else).
Other retailers usually deliver in less than 7 days, I got an item recently from More Computers in a shade over 12 hours.
I've ordered two things from Vistaprint recently, and as neither order was urgent (2012 calendar & xmas cards) selected the Slow Delivery option, both arrived well within the 28 day quoted time.
This isn't limited to online sellers though. I deal with a number of large CCTV manufacturers at work, spending around £250k a month, and delivery times vary massively. Far Eastern suppliers can be forgiven, as they normally wait for a full UK container before despatching, but the US company I deal with has a 4 week delivery lead-time, and this is airfreight for goods in stock.
Two European companies have vastly different policies, one despatches next day for any order, so we get the kit in three or four days, another only despatches twice a week, and then only if we've accumulated €3,000 of order, or we have to wait for the next shipping slot. Once despatched they obviously take the long way round, as delivery takes 5 days from Scandinavia...