Determine Source of Network Bottleneck to Individual User

I’m currently trying to troubleshoot a rather annoying issue with one member of my small community, this user in based in the UK and is having severely limited [potentially throttled] connections when downloading static content from our server.

For some level of comparison, from my server in NYC1, I get about 6MB/s when downloading from our NGINX server; he will get around 500KB/s. If relevant, I have about 31ms ping and he has around 110ms~.

Is there a way wherein I might determine if this bottleneck is indeed his ISP or some other networking issue on his side?

I’m currently using a configuration wherein the traffic is using SSL http/2, and the configuration of the static directory is as follows:

location /storage {
		access_log off;
		
		aio threads;
		directio 16M;
		output_buffers 2 1M;

		sendfile on;
		tcp_nopush on;
		sendfile_max_chunk 512k;
	}

I’ve been tweaking the NGINX configuration to attempt to get a higher throughput for serving large files, but it seems that has had little effect for him. That leads me to guess it’s something else on the server itself or perhaps his ISP.

The server itself is running Ubuntu 16.04, 2gb of RAM, 1 vCPU, and 50gb SSD, and is located in NYC1. It typically has very low load, as we use it for small functions in our small community, like serving files.