Hs65 files are "delivery" files and are hard to handle for any editor. Its like opening a .rar or .zip file and then doing a bunch of photoshop edits on it, its just not optimised at all for editing.
Chucking horsepower at the problem may help, but the nature of the filetype will probably cause problems quite often, even high specc'd machines can have trouble with them. Doing optimised media helps because it creates a file thats not compressed and good for editing, the downside is the waiting and constant clearing of temp cache on your hard drive.
If you're doing a lot of shooting I'd grab a CF card and shoot in ProRes, it wont have the problems H264/265 files have, its a proper editing codec and will be handled much better when editing. CF is an annoying format to shell out on but if we're this deep with the XH2 then you might as well maximise it with ProRes over CF.... or ProRes RAW / Blackmagic RAW via an external recorder.