The picture is blurry
Soft text is usually about pixels, not speed. Work down this list.
Check the dot
Amber means the session is going through a relay, which caps how much data can flow. See direct and relay.
Use 1:1 on your side
⌘2. Nothing is scaled, so nothing is softened. You pan to see the rest. See display modes.
Lower the far computer's resolution
720p or 1080p instead of higher. Fewer pixels means more data for each one, which is the single most effective change.
Switch its quality to Sharp text
It keeps full colour detail, which is what makes coloured text and thin lines readable. See host settings.
Only then think about speed
Check LOSS in the toolbar. Under 2% and the network is not your problem. See the numbers.
Why lowering resolution helps
The far computer is given a data budget by the network. Spread that over four times the pixels and each one gets a quarter as much. A crisp 1080p beats a soft 4K on any link that is not enormous.
Blank window instead of blurry
Frames arriving but nothing drawn, on a Mac or an iPhone, is a colour format the device cannot decode. Set the far computer's chroma back to 4:2:0. See picture quality.