the hashrate is around 1.4Gps
1.4 graphs per second (gps) is the graph rate, not the hashrate.
Grin's Pow is not like most others that just compute a hash function (hashcash).
Instead it's looking for a 42-cycle in a random graph.
See
https://cryptorials.io/beyond-hashcash-proof-work-theres-mining-hashingfor more explanation.
Once a 42-cycle is found, Grin does compute a final cyclehash to see if the solution difficulty is high enough. But on average only 1 out of every 42 Cuckatoo graphs has a 42-cycle. Assuming that this ASIC finds every 42-cycle that exists (fidelity 1), the solution rate of this ASIC is 1.4/42 = 1/30 sol/s, or two solutions per minute. Since Grin has a one minute blocktime, this miner gives you on average two chances to win each block. That's quite different from a bitcoin miner like the S19pro, that gives you 600s * 110Th/s = 66,000,000,000,000,000 chances to win each block!
Note that for Equihash performance, people often specify the solution rate.
Since a single Equihash instance has on average 1.89 solutions, this makes perhaps more sense than using the instance rate. Also, there is not a good name for a single Equihash instance. You'd end up using Equihashes per second.
But for Cuckatoo, with solutions being so rare, it's much easier to quantify the graphrate, with a graph being a single Cuckatoo problem instance.
Btw, a C32 graph is itself described by 2^32 * 2 = 8,589,934,592 siphashes, but an ASIC could avoid computing some fraction of these, and could be computing others several times, so siphash rate is not a very suitable performance measure either.
In summary: hashrate is for hashcash PoW. For non-hashcash PoW like Grin's Cuckatoo or ZCash's Equihash, other measures like instance rate or solution rate are appropriate.
