Yeah.I have tried Evernote on three different computers, and the client is really slow on all of them.
I am a programmer my self, and I am pretty sure you are absolutely correct. The local client is just a shell/ embedded browser, but all the main functionality is running in the cloud. I have my self programmed such an application using the Electron framework. If this is badly implemented (as it seems in the case of Evernote), this can be really slow.
I'm converting all my notes to markdown. All my notetaking including files storage and processor usage will no be local with sync to cloud. Evernote is a real example of what can go wrong with using cloud services.