Hi Salple,
Thanks for reaching out using Apple Support Communities. Here to help.
We understand you are seeing varied network download speeds on different MacBooks connected to the same network.
We wanted to provide a few resources which could help isolate causes for these discrepancies.
This article covers recommended settings for networking devices used with Apple products:
Recommended settings for Wi-Fi routers and access points
Next, you may wish to read over these recommendations for reducing wireless interference:
Resolve Wi-Fi and Bluetooth issues caused by wireless interference
Some of these suggestions will depend on your router. For example, is this a dual band router? If so, reducing the number of active devices using the same frequency band may reduce interference and improve performance.
You may also wish to speak with your internet provider as they may have specific steps to improve performance across multiple devices on your network.
Hope this helps! Take care for now.