What I have seen by monitoring the network is that 0.14.x software has been (so far) the fastest to relay blocks.
Which is understandable as it relays blocks before verifying it.
Just to clarify: they are only sent before being completely verified only for opportunistically sent compact blocks to peers who have specifically requested it. (And they are not completely unverified even then, POW, presence of all the data, etc. is still checked first)
What is not understandable for me, though...
Why pre 0.13.0 software is faster in relaying transactions?
0.14 is "faster" at relaying transactions, much faster in fact, but it has higher _delay_ because the delays are intentional and older versions had a bug that bypassed the original privacy protections for transactions, and also wasted a lot of bandwidth in the process.
I was just thinking, might this lower performance be caused by the memory pool feature, that groups txs so the blocks could be miner for more dosh?
Nah. Ancestor feerate handing is very efficient. It does take a little bit of time, but we have since made the rest of the processing much faster to compensate. E.g. since 0.12 we eliminated _disk_ accesses which always happened for accepting an unconfirmed transaction to the mempool, as you can imagine that was a pretty big speedup.
Nodes are not supposed to relay transactions as fast as possible, doing so leaks the origin of transactions in the network pretty badly. It also wastes bandwidth from sending single transactions at a time in INV packets and from transactions crossing in flight. Unfortunately performance enhancements back around maybe 0.8 or so broke the original transaction relay delays.
In recent versions transactions are queued to be advertised to peers, then sorted by dependency (to eliminate orphans) and fee-rate (so most tx most likely to get mined quickly relay faster), any replaced or evicted or remotely advertised transactions are eliminated, and what remains is sent on a random timer (with a 5 or 10 second expected time, which ends up being 100ms scale for the node in aggregate) and rate limited to a couple times the networks nominal total rate.
I expect that as further privacy improvements happen, the average delay will go up somewhat further. These delays are still negligible in general.
You can read more at
https://github.com/bitcoin/bitcoin/pull/7840 and in the release notes.
Also it's likely that the main reason you see more transaction traffic from old nodes is that they don't implement fee-filter so they will relay you a lot of stuff that you just will discard, like low fee spam.