If there are formula dependencies on the rows, Load in a row and calculate, load another row and calculate, and so on. With complex formulas, this can be time-consuming.
Since moving from Excel to Coda requires adding the formulas, I’ve copied all data over first, then added the formulas after. If you already have a doc setup, you can create a new doc with a new table while keeping the same column names. Copy your data over. Then copy your formulas over one by one. This might be a lesser weight on your browser while you’re working through this migration.
I’ve also had a situation where I just moved 100 rows at a time. Once all data was in Coda though, everything was much easier to work with.