whenever there is a big drop that many people sell or whenever there is a recovery (like now) when all those people and more come back to buy back bitcoin these exchange servers can not handle this much customer and become slower and even go down sometimes. and that is something that we can't do anything about.
even if you switch to other exchanges like Kraken,... you will see the same problem at some point.
Maybe, the best way to avoid the above issue - try not to buy/sell Bitcoin/BCH/Ethereum/Litecoin during the massive drop or rise of a certain coin. Else, we still need to face with the unavoidable website/dropdown menu freeze patiently...