Yes, your ISP can configure your router or modem to prioritize your PC's connection.
Yes, your ISP can configure your router or modem to prioritize your PC's connection.
You can try setting your router to prioritize your PC for better performance during downloads.
Typically you won't find this. You should use a dedicated router for this type of performance setting.
Observe that "focusing on" will allocate more resources to one device while limiting others during simultaneous downloads. It appears this aligns with your situation.