Double Spend Attacks on Exchanges

Thanks for the explanation!
A little addition, the estimated attacking hashrate may be as high as 175MH:

A side effect is the mining difficulty right after the attack is very high so the rest of the miners have hard time finding a block after the attacker leaves.

Miners, it’s important to keep mining so the cost of the attack remains as high as possible.

update 4 hours later:
and right after the difficulty fell, they are back at mining full speed. So basically they are milking the BTG network out of the mining rewards and leave the rest of the miners to mine the hardest blocks. It’s a vicious circle.
The worse part is, this may no longer be considered an attack (unless they still orphan some blocks, but it looks like they stopped) and it may just be a consequence of being a smaller coin amongst coins sharing one algorithm. PoW change may indeed solve this but only if BTG is using an Equihash variation that noone else uses.