I have 112 rows in lastpass, which I use as extension in Chrome and af corse in Lumia 920. I haven't noticed any lag in both. I don't use it over server but it needs always internet connection.
I gave wrong information about pictures. I have added e.g. passport picture, Driving licence etc.
If u use much more protected information u should consider database program with binary fields (pictures, videos and documents).
sorry my English (i'm from Finnland)