This issue is because you must be using 64 bit IE driver service. This causes 64 bit IE to be launched while test run. Now 64 bit has this problem of slow send keys because of the issue described here as mentioned by Lakshay earlier.
Now how to override to 32 bitness? Here is what you have to do
1. Download 32 bit driver service from http://selenium-release.storage.googleapis.com/index.html
2. instantiate your InterExplorerWeDriver class using InternetExplorerDriverService class with path to 32 bit driver service.
This way it will always short circuit to 32 bit service
Just in case you need to know the exact steps
InternetExplorerDriver ieDiver =
new InternetExplorerDriver(“Path to the 32 bit Explorer driver service”);
Make sure that you build your test for a 32 bit system