The game only uses a single CPU core for some reason, whenever I tab into the game core 0 spikes to 100% usage and when I tab out it resumes idling. 

Occasionally when I dash, or almost every time I pick up an item mid-air meant to replenish my dash (gems and such) the game stutters for a few frames, this could be intentional but I REALLY hope it isn't, it makes some sections unnecessarily difficult when you're chaining these dashes together.

I'd also like to know why there's no option to remove the 60 FPS lock in game, the menu works fine at higher FPS so is it an engine limitation? I'm really curious as to why this is.

Really enjoying the game outside these issues, there's bizzare difficulty spikes for some strawberries but I know the assist mode exists for people who have trouble completing everything (truth be told I haven't enabled it once yet out of fear it'll instantly give me some mark of shame styled achievement), which I think is a good inclusion even if I don't plan on using it.

I believe they're shortening that stutter in an update soon.

Yes, the latest version up on itch should now remove freeze-frames with the green refill gems.

The 60fps is on purpose - we lock the framerate so that the game stays deterministic for every playthrough.

Not sure about the CPU core stuff - the game doesn't really do any threading except when loading content.

I would recommend locking your FPS in games to a max of 60 . The massive fluctuations in frame rate can negatively affect your play by having jitters where 200 FPS becomes 70 FPS all of a sudden. The other thing to consider is you want your system to synchronize with the server as accurately as possible. By having an extremely steady frame rate, on  steady network connection, and the right network config you can help to maximize your synchronization over the network, for better hit registration (you click with your cursor over an enemy and the server interprets it as a hit and registers the damage) and so that your teammates don't appear to lag around as much, either.
Typically this can be done through the console, but every game is different.