## 3 Conclusions  
This study presented an in-depth evaluation of the impact of advanced security measures on the performance of the IoT Cloud infrastructure used by UBT Smart City. The system, initially configured with unrestricted communication and no layered security, was restructured to incorporate robust protection mechanisms, including Azure Virtual Networks with subnet isolation, Network Security Groups (NSGs), routing policies, VPN-based access control, and DDoS mitigation. These modifications aligned the architecture with best practices in secure cloud computing for IoT environments. While the M/M/1 model provided useful insights for performance analysis under normal load, future work should explore M/G/1 or G/G/1 models to better capture the variability and burstiness observed in certain IoT traffic scenarios. The mean delay before security implementation was 626.15 \(\pm\) 193.80 ms, and after implementation was 745.88 \(\pm\) 281.62 ms. [PERSON]'s test indicated equal variances (\(p=0.41\)). The t-test showed a statistically significant increase in delay, \(t\)(30) = 4.21, \(p<0.01\). Quantitative results collected from controlled tests demonstrated a clear increase in end-to-end data processing delay--from an average of 626.15 milliseconds prior to security implementation to 745.88 milliseconds after. This represents a 19.15% increase in latency. A two-sample t-test yielded a significant difference between the two configurations, \(t\)(30) = 4.21, \(p<0.01\), confirming Hypothesis 1, there by validating the first hypothesis (H1): _Implementing advanced security mechanisms in the IoT Cloud architecture causes a statistically significant increase in data processing delay_. Despite the increase in latency, the results show that the system remained stable and within acceptable operational bounds. Using an M/M/1 queueing model, we further analyzed how the increased utilization affects average wait time, system time, and queue size. The theoretical analysis aligned well with the empirical data and supported the conclusion that while there is added overhead, it does not compromise system reliability. The financial impact of security controls was also assessed. Although the use of VPN, DDoS protection, and virtual machines introduced periodic cost increases--particularly during days of active system changes--these expenses were predictable and manageable within standard operating budgets. This outcome validates the second hypothesis (H2): _The introduced security measures improve access control and system integrity without causing unsustainable cost escalation_. In conclusion, the study confirms that integrating strong security into IoT Cloud platforms is both necessary and feasible. The performance degradation observed is a justifiable and quantifiable trade-off for the substantial improvement in system confidentiality, integrity, and controlled access. The methodology developed--combining real-world implementation, queueing theory, and statistical inference--offers a reusable framework for evaluating similar architectures in other smart city contexts. Future research will expand this model to include elastic cloud resources, adaptive traffic shaping, and security-performance optimization strategies under dynamic workload. Conceptualization, B.Q. and E.H.; methodology, B.Q. ; software, L.M. ; validation, B.H., L.M. and E.M.; formal analysis, E.M.; investigation, B.Q. and E.M.; resources, E.H.; data curation, G.M. ; writing--original draft preparation, E.M. and B.H. ; writing--review and editing, E.H.  
and G.M. ; visualization, L.M. ; supervision, E.H.; project administration, B.Q. ; funding acquisition, G.M. All authors have read and agreed to the published version of the manuscript. This research was funded by the Bulgarian National Science Fund, Grant No: KP-06-N52/7, 2021. The project is titled \"Mathematical models, methods and algorithms for solving hard optimization problems to achieve high security in communications and better economic sustainability.\" The APC was funded by the same grant. Not applicable.