Since the question specifies browser and not client, my vote is for D as first step in troubleshooting.
Identifying Browser Performance Issues
Performance issue could be caused by the browser you are using. Use the following steps to verify whether this is the case.
To verify whether a performance issue is caused by the browser:
Check the browser version.
Verify that the browser version is supported by NetSuite. For more information, see Supported Browsers for NetSuite.
Ensure that you are using the latest version of the browser.
Do not use trial, development, or early adoption browser versions.
Check whether the problem is occurring when using other supported browsers.
https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_0825025054.html