Since a recent update to chrome, the presets are no longer labelled with bandwidth.
Chrome used to list the actual speed of each one so you could simply tell.
What bandwidth or latency do the options here represent?
I did some measurements with two speed tests available in the internet. With the following custom profile I received similar download speed and ping latency as with the presets.
Slow 3G Custom: Download 376 kb/s, Latency 2000 ms
Fast 3G Custom: Download 1500 kb/s = 1.5 Mb/s, Latency = 550 ms
The actually download speed measured via the speed tests was only slightly below the configured values. The measured ping latency was half of the value configured in the custom profile.