Hey GiapCuong!
There have been some great comments by other community members so far, but I also just want to let you know that if you're using the Dashboard throughput tool it can give you potentially unreliable data.
More info on why can be found in this doc:
https://documentation.meraki.com/General_Administration/Tools_and_Troubleshooting/Throughput_test_to...
To summarize. The dashboard throughput test can show lower speeds than you'd expect because it uses Meraki management traffic, which has the lowest priority compared to your production network traffic. Also, it's only testing throughput to the Meraki cloud, which may be different from your actual client throughput out to the internet.
What results do you get if you just run a normal throughput test using something public like speedof.me on a wireless client?
-Donovan
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