A few recommendations to improve performance:
#1) %TimeStamp is a nice thing for display tough rather inefficient for indexing as it is mostly close to a unique index.
If you just check for Date as you describe you better use an index on a calculate property of type %Date
instead of a string, your date values in index are just integers !!!
that makes your index slim and fast.
check for = is the easiest one, therefore, the fastest
check for > is the slowest as you have no stop condition
between is somewhat better as you have to check > and < but you have a start / stop delimited range.
And integer compare is by dimensions faster than any string compares with any available processor chip.
And you do lots of them!
in reality, better check the number of Global references and lines you execute.
That's the truth. Costs are good guesswork based on generic parameters out of tune table but rather for the dimension.