QB Issue Resolution:

gear>settings>company settings

left side menu, form delivery>customize forms

scroll down and uncheck include account summary

if it works better, it is not your mouse or keyboard, it is the intuit data lookup slowing things down

Resolution for Issue 'when invoicing, the loading of summary for customer takes a long time, can i stop this feature?' available: Yes (Solved).
Source: Intuit Community forum.

when invoicing, the loading of summary for customer takes a long time, can i stop this feature?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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