With high amount of data in your tables - query execute time is slow and which means your app becomes slow.
There are couple of tricks that can be used to increase speed : -
If insert query is slow - use skipDataCheck to speed up the insert query.
If select query is slow - try to find different version of your query & compare the time. Use IdbStudio for these purposes.
Note :- IndexedDb is a client side database, which means a client can be a high configuration pc device or a low configuration mobile device so fill up your database according to your client.