I would like to make a sepparate post to address concerns regarding removal of POW as a spamm prevention mechanism.
Although we can all aggree that POW doesn't provide the necessary spamm mitigation, it provides a barrier for average users in spamming the network. You have to put a minimal amount of electricity in order to use the Nano network. Keeping the cost of the transaction out of the network is what makes Nano brilliant from an economic standpoint.
Removing POW will make all tiers/buckets susceptible to spamm without any inconvinience on the spammer's side. While this spamm will not be as dangerous for QoS as the current one is due to transaction prioritization mechanisms, bloating the ledger will be easier than ever.
Let's say that Nano can operate at 100TPS and that node operators set a bandwidth limit that allows such performance. We can assume that all the lower tiers will be constantly spammed because the cost of spamming is ~0 and the middle tiers, which are used most often but require a small financial investment, could also be spammed by sending a minimum tier specific balance in circles. Only the few high level tiers would be spamm free and they optimistically would take 20TPS of total 100TPS.
If every transaction could be used to open a new block, then we have a ledger bloat attack on our hands with 80 new accounts being opened every second. Almost 7 million new accounts-per-day can be opened without taking a single dime out of the attacker's wallet.
I'd like to emphasize, this isn't a discussion about performance issues, this is a discussion about ledger bloat resulting from POW removal and ways in which it can be mitigated.