How much faster would the network need to get, in order to meet (or at least approach) the speed of a local SSD? are we talking about needing to 2x or 3x the speed, or by factors of hundreds or thousands?
Networked storage negates that significantly, absolutely killing performance for certain applications. You could have a 100Gbps network and it still won't match a direct-attached SSD in terms of latency (it can only match it in terms of sequential access throughput).
For many applications such as databases, random access is crucial, thus why nowadays' mid-range consumer hardware often outperforms hosted databases such as RDS unless they're so overprovisioned on RAM that the dataset is effectively always in there.
The problem is that this is not possible when the next IO request depends on the result of a previous one, like in a database where you must first read the index to know the location of the row data itself.
10GbE is about the best you can hope for from a local network these days, but that's 1/5th the bandwidth and many times the latency. 100GbE would work, except the latency would still mean any read dependencies would be far slower than local storage, and I'm not sure there's much to be done about that; at these speeds the physical distance matters.
In practice I'm having to architecture the entire system around the SSD just to not bottleneck it. So far ext4 is the only filesystem that even gets close to the SSD's limits, which is a bit of a pity.