I would first need to see some data on the mesh link, but a couple of things:
* The 2.4Ghz radio on the mesh point, is the RF clear and not congested? What about the 2.4Ghz on the portal?
* What is the mesh link quality? You will have to get into the CLI of the mesh portal and point and run 'show ap mesh link' from each side and post the output
* If you have the SSID being broadcast on the 5Ghz as well as 2.4Ghz, it will slow down the link throughput on the mesh "considerably". Even more so if there is a client on either the point or portal's 5Ghz radio. The best thing would be to make those two APs broadcast the SSID in 2.4Ghz only and preserve the 5Ghz for mesh only.