Checking out the block explorer and am curious about the below:
"Seen time of newest mature block Oct 10, 2015 at 8:52am (UTC)"
This obviously can't be the last found block as it's almost 4 days ago. If its the amount of time before you can spend you coinbase transaction then isn't that rather a very long time?
That date is the last mature block, or as you said the time before you can spend your coinbase.
Block times have been pretty slow lately so it may be artificially long.
Correct, it's the most recent coinbase that can be spent. It's supposed to be 24 hours, but thanks to heavy mining instability (we've got a pool that hops on and off as the difficulty adjusts), our median block time is closer to 40 minutes.
The heavy mining instability is the consequence of the difficulty adjustement : the block time is directly linked to difficulty (the higher the slower) and hashrate (the lower, the slower). The mining profit is directly linked to the difficulty (and price!).
I noted the following pattern since about 10 days :
step 1 :diff = 100X : at this time : very high --> slow block production because diff is high and hashrate is low since it is not profitable or not the most profitable coin : about 1 block per hour or worse
step 2 : 500 block later (a week or more) : diff is adjusted : 50X (half the previous one) --> about normal block production : hashrate increase since it is profitable, block time seems about normal, some farms and miners are back
step 3: 500 block later (two days) : diff is again adjusted (why?) : about 16X or 25X (half or less thant the previous one): fast block production with huge hashrate since it is the most profitable coin, private farms jumps with full power.
step 1 again : 500 block later (less than one day) : back to 100x step 1
As you may see we are locked in this circle. To my opinion, difficulty adjustement needs some tweaking for smoother block production : even a public pool may not be enough to fix this (but it will help).