I use excel, don't see any issues with it as I know Excel very very well.
To me, the data is the same. In Excel for my set inventory, it is probably 1,000-2,500 rows long but each year I clear out anything that sold to start with my beginning inventory for the year again. Then add anything newly acquired afterwards so I have past year and current years separated on the same tab, but then once again, will merge the data, sort it, and repeat.
I also put every bricklink sale on there too. I know BL has a downloadable spreadsheet to give you this but I have my own which also calculates all fees automatically and all I need to type in is actual shipping cost. Since I deduct a portion of inventory with every piece that is sold, it works great for that too to track my inventory deduction from my parting out side of things too.
Another tab for all direct expenses and another tab for mileage.
Then a final tab in the front that summarizes everything for me in terms of all revenue broken out nicely, all expenses, and all inventory purchased and sold.
To the OP, if you excel is "slowing" down, I would suggest an upgrade of your computer parts. Excel shouldn't slow much down unless the file is absolutely huge.