The "Average Holding Time for Incoming Messages" managed SIP stack performance counter seems to be affected by the time that it takes for the user to answer a call in Microsoft Skype for Business Server 2019 or 2015. This performance counter is supposed to measure how long it takes to make a routing decision about CSIPMessage instances. It should not depend on message timing.
To fix this issue, install the July 2019 update for Key Health Indicators for Skype for Business Server 2019 and 2015.
This update provides the fix on the server side. It only applies to Skype for Business Server 2019 and Skype for Business Server 2015.
There’s also a fix in the Key Health Indicators (KHI) tool itself.
No server fix for Lync Server 2013. You only need to apply the latest update of the KHI tool for Lync Server 2013.