Why is my internet slow?

A single Mbps number cannot explain why an Internet connection is slow. Throughput, latency, jitter and packet loss describe different properties, while Wi-Fi, Ethernet, competing devices, VPNs, routing and the test server all affect results. High throughput can coexist with poor latency under load.

How to find the bottleneck

Separate the LAN from the ISP path: check signal or cable, then ping while idle and under transfer, then throughput. Repeat measurements under controlled conditions. A problem that occurs only in the evening, only through a VPN, or only toward one destination points to different bottlenecks.

The detail that matters

Compare idle latency with latency during a saturated upload and download, not just headline Mbps. Also compare Ethernet with Wi-Fi and one destination with another so local radio contention, ISP congestion, VPN overhead, and remote-server limits are not mistaken for each other. For a control measurement, use 2ip speed test.