Any updates on this bug? I have the same issue with my MacBook Pro running OSX El Capitan. I can use Evernote for some time, but then it starts to slow down. Checking the Activity Monitor, I noticed that Evernote can consume up to 4 gigs of RAM. Restarting the program takes the memory usage to around 170 megs.
I'd suspect there's some kind of memory leak in the program related to inputting text. Sometimes writing pauses and copy paste can also cause some hangs. Actively writing also brings the CPU usage to strangely high levels.