I'm a user in China and I am using the yinxiang.com service, troubling with the same lag.
I experience this when the note is syncing after a long time offline (or just not be synced). I have submit several tickets on the website and feedbacks via wechat about this problem, with nothing changed. Maybe this forum post could help.
I guess the problem is that the sync network APIs are called in the UI (main) thread and when waiting for the response, they blocked the thread. So when there is no network API called, no lags occurred. That is why my evernote lags only when syncing and the firewall bans will work.