[1.5.4] Performance improvements when working with large amounts of data

Currently, the app works fine if you add a typical number of transactions each year (typically means that there are about 300 transactions per year, so after 10 years, for example, you would reach a total of 3.000, which the app can already handle today without any problems).

However, some users want to import all their existing data of the previous years into the app using the CSV import (sometimes 10.000 transactions and more / over 2.500 transactions each year). But as soon as they do this, the app slows down considerably.

Since in the long run it is likely that multiple users will have thousands of transactions stored in the app, the issue should be investigated and fixed.