Bonus gold for killing a beserker won't be that wrong. It isn't the most easy thing to do, because of the spawn rates of scrolls / beserkers and the scroll duration.
Lag? Toggle quality when in the gathering sequence, there is a menu button. At this menu you can also toggle the sound. It's semi-hard to find, but it is there ;)
The speed curve is too steep. If you calibrate this to be less annoying. What I mean is that at later levels, if your speed is to high compared to the damage/defence you cause/have your char can glitch at the end of the screen making it impossible to go through with the level without clicking rapidly instead of just holding the button.
Tl;dr, your speed-curve is to steep, alter this please :)
Soon(ish). Probably in the next update.