Is there a limit to the amount of data I can have in QuickBooks Online?
No, there isn't a file size limit in QuickBooks Online.
Does the amount of data I have on QuickBooks Online affect performance?
QuickBooks Online loads your data as needed, which means the overall size of the data file doesn’t substantially affect performance. In addition, QuickBooks Online uses a processing method where some of the processing is performed online and some is done locally using your device processor and memory. Lastly, the speed of your internet connection plays a big part. Meeting or exceeding the System Requirements for QuickBooks Online improves QuickBooks Online performance more than file size.
Are there limits on lists, such as accounts, customers or items/inventory?
Using the same “as needed” approach listed above, large lists don't cause performance issues. You don't have to worry about entering too many customers, accounts, or items.
Are there limits on the number of transactions? Can transactions affect performance?
There are no transaction limits. As far as performance is concerned, transaction complexity has more impact on performance than transaction volume. For instance, an invoice with a single item and no sales tax is less complex than one with multiple items and with sales tax applied. But unless you are processing a large volume of transactions that are also very complex, the effect on performance should be minimal.