This is the question that you want to ask: "What is the difference between 'too slowly' and 'very slowly'?" And here's the answer: 'Too slowly' means 'not fast enough'. This is bad - it means that the lack of speed is a problem. For example "He lost his job because he did his work too slowly" or "He missed his bus because he walked too slowly". If the minimum speed needed to get somewhere in time is 5 mph and you go at only 3 mph, this is "too slowly" - in other words, insufficient speed. 'Very slowly' just means 'at a very slow speed'. It can even be a good thing. For example "I like listening to children's stories on TV because the presenters speak very slowly, and so I can understand them". I hope that helps.

The simple asnwer would be that, He's walking too slowly - he's walking so slowley that it is a problem. He's walking very slowly - this is more neutral, it could be a problem, but it could not be a problem as well. :)