Air Navigation Service Providers (ANSPs) play a critical role as a natural monopoly within the air traffic system and are subject to regulation. Achieving preset performance targets necessitates efficient resource planning, contingent upon accurate traffic forecasts. This means that forecast precision is a key determinant of operational efficiency. In this study, we employ backcasting techniques to gauge the influence of forecast errors on air traffic management performance. This is done for eleven airspaces and seven years of data. The paper seeks to estimate the cost of delays if the actual number of flights for the period 2015-2020 had been as predicted by EUROCONTROL through its specialised service STATFOR. To assess the impact of forecasting errors, we analyse the discrepancy between the predicted and actual figures for flight data, specifically focusing on delays. Our results show that forecast errors have a noteworthy, adverse effect on performance. Inaccurate predictions prevent efficient resource allocation. We prove that a marginal increase in forecast quality would significantly reduce overall costs for stakeholders.