It is normal to slow down as the no of records in the database increase. If possible, you may try creating two databases. One database with all the recent data(say past 3 years) and other with old historical data. When you doing routine analysis, you can use the smaller database, and when you are doing some task that needs thousands of records, say backtesting, you can use the other database.
Other than that there may be routine PC issues. Try disk defragging, free up some disk space, etc.