I agree with @kiharajon's theory that they're concentrating outside Nairobi. I'm in Ngong and my phone never loses 4G coverage indoors or outdoors. Open signal shows that I have three Telkom 4G boosters all within 1 km of my house. I kid you not, the 3G at my house is weaker than the 4G one. I've never seen 4G lose a single signal bar but 3G loses the highest one quite frequently. I just did some speed tests and here are the results. I'll keep editing this post and changing the screenshot when I do more tests. I'm doing these tests on my 4G4Free allocation coz a single test eats like 120MB in 30 seconds. For the first test the server is hosted by Zuku and the second tested a server hosted by Telkom Kenya.
Edit: Just did another test. This third one on a server hosted by Liquid Telecom Kenya.
One more speed test for good measure to see how things go during rush hour. This time it gave me a server hosted by Airtel (I didn't expect this at all!) Anyways, just as I expected, it looks like download speed goes down a bit but upload speed holds. I'm seeing a trend here - it looks like Telkom is throttling upload speed but at least it's a number you can rely on.