Why Is Speedtest Upload Test Disrupting Pings?

We have been recently blessed with Gigabit Fiber connection from AT&T. In my unscientific attempt to gauge connection stability I usually run 100 DNS pings and SpeedTest - all via latest Win 10 connected directly to router by short Cat 6 (gigabit) ethernet cable.

Today I noticed serious slowdowns in Ping but ONLY during Upload portion of the Speedtest, which itself reported 700mb/s. While 940 mb/s Download test has almost no impact on ping timing. Any idea what may explain this?

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It starts around same time as speedtest started, and at the end you can see "normal" results when speedtest is finished. You can see that download test reported higher speed and had barely any impact on ping test timings.