They recently mentioned these issues:
https://poloniex.com/press-releases/2017.05.16-Industry-growth-and-its-effect-on-Poloniex/

They have had a huge influx of members come into the site. Since ETH spiked, the site went from being used on a regular scale to what it is now. I personally never imagine it becoming this big. Give them some time to adjust the system and get more servers. it sucks for the people who lost money due to this, but it is mentioned in their terms of service that it is not their fault if something like that happens. It would be nice to see more exchanges being used.