The only thing I'm seeing is the way that you populate your @splitTimes table variable. I'd change it to a set-based method:

-- See Jeff Moden's article

-- The "Numbers" or "Tally" Table: What it is and how it replaces a loop.

-- at http://www.sqlservercentral.com/articles/T-SQL/62867/.

-- NOTE! A permanent tally table will always be MUCH faster

-- than this inline one. See the above article to create your own!

INSERT INTO @splitTimes

SELECT [Time] = DATEADD(minute, N-@interval, '00:00')

FROM Tally

WHERE N <= 1200 + @interval -- # of minutes to span 00:00 to 20:00

AND N % @interval = 0; -- allows any interval desired

Edit: Actually, I do see another thing. Your where clause. By using a function on the dates being searched, you are eliminating the use of any indexes that might be available. Rewrite them so that they are in the format:

WHERE sess.sessionstart >= <your starting date>

AND sess.sessionend < <your ending date + 1 interval)

In the case of what you're using:

WHERE sess.sessionstart >= @day -- 20101122 00:00

AND sess.sessionend < DateAdd(hour, 20, DateAdd(minute, 1, @day)) -- 20101122 20:01