1.45 to 2.65. It was almost double, and 80% were asics.
https://etherscan.io/chart/difficulty
Also to prove my point, profits crashed and network still the same cause gpus miners are leaving and many left the network and to camouflage it, bitmain and co are putting asics to cover the hashrate of those gpu miners that left/leaving the network. They don't want it to look suspicious.
Given the continuing extreme shortage of AMD RX570/580 GPUs despite their announced ramp-up, and the even worse shortage of Nvidia 1060/1070 cards in the timeframe of that hashrate climb, and that it did take months to almost double, I doubt VERY MUCH that a significant part of it was ASIC.
There was ZERO REASON for GPU miners to be "leaving" at that point, as profitiablity was still quite high on ETH, since it's PRICE has weathered the "cryptocoin downturn" a lot better than most coins INCLUDING BITCOIN.
I'll also point out that the profitablity compared to ZEC and it's offspring was climbing quite a bit, which would have led to some ZEC miners pointing 1070 and 1060 ZEC rigs at ETH instead (I'm ONE of those on 1070 GPUs, so don't bother saying "it didn't happen").
There might be ETH ASIC out there, but not in the kind of quantity you seem to believe, or they're very low "efficiency gain" over GPU miners and no major threat to GPU mining.
I'll also point out that once the price drop on ETH got serious, ETH total network hashrate flattened out - not DROPPED, just stopped growing much - which ALSO makes it apparent there's no significant number of ASIC being built.
There's also the question of "where are the USED GPUs" that your ALLEGED "gpu miners that left" would be selling off - no signs of them anywhere I can see, and since AMD GPU new pricing is not dropping no indication of a significant number of them ANYWHERE.
1050 isn't a good ETH mining card - it does better even now on ZEC - but the 1050 ti does better on ETH.
Even now the Vega is better off on XMR than ETH.