Low fps when viewing through Webpage

The client performance can not be the problem, because a single cam has sufficient FPS.
The server is using hardware acceleration. Switching it off is no option (due to overall performance). I tried lower resolution and other streaming profiles (superfast, 25%, ...) with no big differences.
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Do you have the same issues with UI3?

PS: UI2 has the same issue.
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Any idea what might be wrong with my setup?

PPS: same issue using local web UI (-> no network issue)
BI version: 4.7.3.0 x64 - Issue is browser independent Chrome, Firefox, Edge, Yandex

were you ever able to fix this?