What kind of business are you running and how do you record, for example, Customer Invoices, Vendor Invoices, payments from customers or vendors?
Usually, as Business Accounts, one has at least one each of Customer Invoices (A/R) and Vendor Invoices (A/P) and an optional *Sales Tax* account.
For a test, create a copy of your Quicken data file.Using the Copy, create accounts of type Accounts Payable and Accounts Receivable.Now, when you switch to the Business tab, how long does it take to open?
If it's fast, continue using the copy and abandon the original file.
Your suggestion worked! I created a dummy Vendor Invoice account, and now the Business tab updates quickly. Thanks for your help!