Your strumming hand?
Ecchan uses a capo for this song (when she performs live) which might make things easier to fret. (Roxy tabbed standard tuning).
But if not being able to strum fast enough is the issue, the capo won't help.
I don't think the strumming needs to be lightning speed for this song. It's probably just a little faster than Shangri-La for example (comparing the fast strumming parts).
If you watch Ecchan perform Hira Hira on Music Station, the strumming cadence doesn't seem too dificult to maintain.
It is a bit of a workout though, but I feel more sorry for Kumiko.
I tried playing this song cuz it's an old favorite with simple 5th chords
... but then I gave up. Too fast/tiring.http://youtube.com/watch?v=QoQAZDdl4LY
I don't think I'll ever be able to play this song:http://youtube.com/watch?v=22kMwHSgQrk
edit: I think I get what you mean Kasumi. I find it difficult to strum the first chord consistently fast w/o accidentally hitting E1 or A5.
Maybe adding the capo adjusts the tabbing so the speed of the strumming isn't changed but the strings plucked are? ..... probably not.
i.e. strumming fast and cleanly strings 1 to 3 is a heckuva lot easier than strumming 2 to 4. I'm not sure if adding the capo accomplishes this but you're right I do find it tough to strum this song fast in standard tuning.
I also noticed that when I strum super quick and sometimes just the root of the chord is heard, i end up playing the bassline for the song at the same time. hahaha.
edit2: I found 2 things that also helped. Switching to a thin pick (you probably already use a thin pick though) and not even looking at your strumming hand to worry about hitting E1 or A5. Eventually you just get used to restricting the strumming motion to the right strings and being able to play super fast.