For me, and maybe this is just my organization, but the new O365 OWA interface is extremely slow. Latency is so bad that typing results in only 1 in every 4-5 keystrokes not failing to register.

If I pretend my user agent is from circa 2012-2013 (e.g., Internet Explorer 10), it will default to an older interface that has marginally fewer features and is much, much more performant.