Search implementation using ASP.NET MVC 2 / jQuery (Performance & Security concerns)

Question

Happy Halloween everyone, hope it was a fun night!

I've just implemented an AJAX search functionality on my first ever ASP.NET MVC project, and I just want to get your feedback on security and performance.

The project is going to be a simple Forum, with roughly 40 topics and 1000-3000 total posts.

Let me show you some code.

In my TopicsController.cs file, I've created a JsonResult action that looks like this:

public JsonResult Search(int id, string title, string keyword)
{
    var searchResults = from p in topicRepository.SearchPosts(id, keyword, 10)
                        select new
                        {
                            Title = p.Title,
                            ReplyCount = p.ReplyCount,
                            CreatedBy = p.CreatedBy,
                            LastUpdated = p.LastUpdatedFriendly,
                            Replies = p.ReplyCount,
                            Views = p.Views
                        };

    return Json(searchResults, JsonRequestBehavior.AllowGet);
}

The topicRepository.SearchPosts() is quite simple

public IQueryable<Post> SearchPosts(int topicId, string keyword, int limit)
{
    var topics = from p in db.Posts
                    where p.TopicID == topicId && p.Title.StartsWith(keyword)
                    orderby p.Title ascending
                    select p;

    return topics.Take(limit);
}

So basically, I generate a JSON object based on a topicId and a keyword that gets passed via the $.getJSON() method.

$.getJSON(
    url,
    { id: id, keyword: val },
    function(json) {
        console.dir(json);
    }
);

The id comes from an Url.Action method while keyword is a string that gets it's value from an input box on the keyup() event. I delay the request by about 300ms to make sure too many requests aren't sent to the server while the user is typing.

The JSON comes back successfully and I build my table appropriately with some nice animations etc.

Now, since this is my first ever ASP.NET MVC project I have obvious concerns about security and performance.

1. Could the code be optimized for faster results?
2. Should I use caching of some sort to improve the speed?
3. By fiddling with the data sent to the server, could a malicious user do some damage?
4. Should I be doing anything else differently?

I don't expect answers to all my questions but any help is highly appreciated.

Thanks in advance,
Marko