From what I can see we've seen a lot of blocks today that have been mined by pools that are using the default 750,000 byte block soft limit (these show up as 731 kBytes), while a few have been mined by pools that support the 1M bytes block limit (these show up as 976 kBytes).
Now what's actually happened is that for quite a number of hours the network was having an "unlucky" day and didn't find many blocks. It wasn't hugely unlucky (there were no hour+ gaps between blocks as happened 3 times in 24 hours a few weeks ago), but it was moderately unlucky.
Starting at 11am GMT these were the numbers of blocks per hour: 3, 3, 2, 4, 4, 8, 5, 7, 8, 4, 9. Over the 11 hour period we had 57 blocks whereas we might nominally expect 66. 57 out of 66 is actually surprisingly common. Much less common (but not totally unexpected) is that in the first 5 hours we only had 16 blocks instead of 30. Again it's not totally unexpected but it's less common. The next 6 hours saw 41 instead of 36 though.
Now total transaction volumes for the day aren't anything unusual (they're steadily climbing but Saturdays are always quieter than during the week), but it seems that there have been some reasonably large size transactions today as the mean block size is 0.424 Mbytes. What that means is that we had a period of time where for 5 hours the transactions could back up. I ran some estimates for the effective network block capacity a few weeks ago and it's around 770,000 bytes per block, but if we're only finding 16/30ths of the normal blocks in our 5 hours then that drops to an effective capacity of nearer 410,000 bytes.
What this means is that for 5 hours we were seeing more transactions than the block selectors were prepared to process (not the network, the miners, or more precisely the mining pools!).
What's interesting are the comments that larger fees didn't help. This suggests that some block makers may not be selecting transactions based on their associated fees. Of course no fee, no matter how large, can make blocks be found any quicker so if there's a 40 minute gap between blocks then you still have to wait 40 minutes.
If anyone's not already seen them, here are the more formal write-ups:
http://hashingit.com/analysis/33-7-transactions-per-second
http://hashingit.com/analysis/34-bitcoin-traffic-bulletin
http://hashingit.com/analysis/39-the-myth-of-the-megabyte-bitcoin-block
http://hashingit.com/analysis/41-waiting-for-blocks
Topic: The network seems stressed (and behind) right now - page 2. (Read 2357 times)
We are not far off the 1M max block size now. The max block size solution Gavin proposed should gain more support.
Well, daily transactions is over 100 000 now ... all days ! 
There don't even seem to be an excessive amount of transactions in these blocks that are approaching 1 meg.
Perhaps someone found out how to create really large transactions and clog up the network?
Perhaps someone found out how to create really large transactions and clog up the network?
I doubt that large (kB) transactions are that hard to figure out how to do. I doubt it's malicious.
There don't even seem to be an excessive amount of transactions in these blocks that are approaching 1 meg.
Perhaps someone found out how to create really large transactions and clog up the network?
if that's what is happening we're all doomed , we need to make the dev's aware of this someone should contact them :> Perhaps someone found out how to create really large transactions and clog up the network?
We tried, but the fees included with our requests weren't high enough to warrant their attention.
There don't even seem to be an excessive amount of transactions in these blocks that are approaching 1 meg.
Perhaps someone found out how to create really large transactions and clog up the network?
if that's what is happening we're all doomed , we need to make the dev's aware of this someone should contact them :> Perhaps someone found out how to create really large transactions and clog up the network?
There don't even seem to be an excessive amount of transactions in these blocks that are approaching 1 meg.
Perhaps someone found out how to create really large transactions and clog up the network?
Perhaps someone found out how to create really large transactions and clog up the network?
Hmm. Seems like the network is really stressed. Too much transactions "clogged"?
Yes, loads of TX waiting for a confirmation lately. Even those with a fee
This is a serious issue needing a solution.
Yes, loads of TX waiting for a confirmation lately. Even those with a fee
The next block came out at 976kB
It's been a while since a block hasn't come out below the 731kB softcap. 
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