RTCOutboundRtpStreamStats.qualityLimitationReason - Web APIs
the amount of time the encoded media has had its quality reduced in each of the potential ways that can be done can be found in qualitylimitationdurations.

RTCOutboundRtpStreamStats - Web APIs
qualitylimitationdurations a record mapping each of the quality limitation reasons in the rtcremoteinboundrtpstreamstats enumeration to a floating-point value indicating the number of seconds the stream has spent with its quality limited for that reason.