I'm no expert but that's pretty big for a sencha app. We've run into some of the same types of issues with clients wanting way more info that should be available on a mobile in there.
We ended up storing the info in the HTML SQL db and querying them out as we need them rather than trying to put all the records in a sencha store and helped a lot. Then when they added more records, discovered putting indexes on the tables made a huge improvement as well.
Haven't tried the SQL proxy in 2.1 yet but that might be another option. We also tend to avoid the tree store since it's rather awkward and use our own containers with back buttons and such to get better control over the experience.