What is proxy speed?
That's the "speed" of the proxy - how fast it uploads/downloads information to/from you. You use the same words for a fast or slow website - if the website loads slowly, it has a low "speed", if it loads quickly, a high "speed".
Think of it this way:
Say your internet has a speed of 20MBps, but your proxy only has about 5MBps speed (the "proxy speed"). If you're using that proxy, you'll only go the lowest speed of the two - 5MBps in this case. You can never go 20MBps because the proxy just doesn't go that fast. This works in reverse too; if your proxy has a speed of 5MBps and your internet is 1.5MBps, then your internet is only going to go 1.5MBps, despite the fact that the proxy is faster than that.
That's my understanding of things, hope that helps.