AppSheet can scale to handle millions of database records, provided the app is set up correctly.
By default, AppSheet downloads the entire spreadsheet when the app is opened, so the app can be used offline. Security Filters can be used to filter the records to just those that are needed in the app. There are many ways to filter the data including filtering old data or filtering by region. User Settings allow users to specify some information (e.g. their region) that can be used in Security Filters to only send them the relevant data.
To improve sync performance on large datasets, a database can be used instead of a spreadsheet. Apps function the same whether using a spreadsheet or database. Refer to the data sources document for information about databases (MySQL, SQL server, etc).