I have received a request for recommendation from an ex-employee on LinkedIn and I don't know how to respond.
I have worked with him for about 9 months. His track record was not very good before he joined my team and to be honest, though he improved a bit during the 9 months he was still a low performer. That is why when higher management demanded cost reduction he was one of the first to go.
How would you respond in such a case? Did anyone have a similar experience before?

Ignore It
I would not write a recommendation under such circumstances. Imagine being cold called as a reference for that person--how would you respond?
I believe the MT advice is to write recommendations, or be a reference, only for outstanding performers.
http://www.manager-tools.com/2008/12/how-to-provide-a-reference
Especially on linkedin
I've seen recommendations on resources that I didn't have a lot of respect for. Made me loose some of the respect for the person who wrote it. LinkedIn recommendations are seen by the world.
Thanks
Thank you for your responses! I am now convinced that I shouldn't write any recommendation in this case.
I have an additional question though. Would simply ignoring it - as in no response - look unprofessional in this case? His request was personalized, not an auto-generated one. I have recommended some other people on LinkeIn and we are still on good terms, so it seems to me a bit cowardly not to respond at all.
Do you have any suggestions as how to politely decline this request?
Declining to Recommend
Summer,
You could thank them for regarding you as a potential reference--since the request was personalized.
While you did have the chance to see their work first hand, 9 months is a fairly narrow window. State you are less familiar with their entire background and that someone else may have deeper knowledge of their work history.
Great advice
Thank you TL for your great advice. This is indeed the kind of answer I was trying to find.