Irregular lag with previewing in browsers

I don't have a lot of objects on my current demo (4 cloned enemies and one player character). I have only a physics object as a weapon.

When previewing it in Chrome, sometimes the spear's movement lags, other times it works smoothly. They are the exact same layout refreshed multiple times in order to get it with no lag.

I have no idea what's causing it. In Construct 2, it says my current layout just uses 10mb of memory currently.

Anyone else gets irregular lags? I need to know exactly how fast or how laggy the weapon is or else I can't adjust my game properly.

I can try uploading/exporting to web for people to preview on other browsers if that would help.

Always share a .capx, it lets us see what you've done.

Okay, the event sheet's a little of a mess but hopefully you can see where stuff goes where: dropbox.com/s/vf1mzigf3y4r38a/SpearDeer.capx

Basic controls:

Arrow keys to move around

Hold Q for a bit to fire the spear (currently only facing right and isn't mirrored even when the player is facing left)

The fired spear is different from the one pinned to the player sprite. Currently it spawns a duplicate physics-Spear.

For me, I'm using Chrome to preview. Sometimes I refresh the browser itself, other times I go back to Construct and launch another preview.

The lag only seems to affect the speed of the path of the Spear, the only physics object. Other things like the animated Deer, don't seem affected.

Edit: Just updated the file. More physics objects that are pinned to the Deer to make the spear hits register. Doesn't change frequency of preview lag (in fact feels less than before, funnily enough).

PS: Why does the forum session frequently 'expire' before 1min and I have to copy and paste my new post and post after refreshing the page within 5 seconds :S?