The primary reasons for slow download speed can include network congestion, limited bandwidth, outdated or malfunctioning modem/router, multiple devices sharing the network, or background applications using bandwidth. Other factors are high latency, ISP throttling, device-specific issues like outdated software, and large cache or background processes on your computer. To improve download speed, try rebooting your device and router, disconnect unnecessary devices, clear cache, avoid multiple simultaneous downloads, update your device and router firmware, and check your internet plan and ISP service quality.