remember the 6 hour lags on mtGox?

40 minutes... pffft ;-)

EDIT: maybe not network congestion, but trading enginge congestion (on db level) could be an explanation. If not coded well, the sell side of the order book being constantly flooded with new entries could lead to lock escalations. As to why your small buy order went through without hassle... maybe they have separate db tables for BID/ASK.
I don't know.

Anyhow, such technical problems should not occur in a trading engine of any reputable exchange. If it was built for trading magick the gathering cards I could understand, but surely the kraken engine would have to have been built with considerable load in mind.

40 minute lag is ridiculous, as I said in 2011 regarding gox' engine: I could probably build one with higher capacity running on my phone.

For perspective, look at some altcoin trading engines like poloniex. Swarmed with bots these engines express no considerable lag at all, neither do the web frontends.

Even if the engine has unfixable throughput problems, this could be tackled at the incoming end: new orders should be denied by API/web frontend if they can't be executed in a timely manner. Everything else is borderline criminal.