Better to identify at what interface user is facing low NR throughput.
In NSA 3x scenarios, Throughput of 5G user = throughput of 5G part + throughput of LTE part.
If no radio issue at NR side, then increase in LTE user/traffic and load, throughput of LTE will degrade and overall user experience will degrade. As a result, NR user throughput/experience can be impacted by LTE part, and the LTE throughput of a 5G user can be monitored by User Downlink Split Average Throughput (NSA DC).
DL ENDC/throughput split is dynamic based on X2 and Uu interface, if LTE buffer is high, the data split to LTE will be less ( this though will degrade 5G user experience but won’t reflect in KPIs or RLC/MAC layer of NR, you have to catch it through PDCP layer).
Also if shared TXN b/w LTE & NR this can also be a major bottleneck.