Just some tips that helped me in game.
Warmup before trying to set records
If you're still trying to reach 60 seconds on hard mode to unlock expert, or you're just going for a record time, you may find it useful to warmup your fingers before trying to set a record. I find that running warm water over my hands for about a half minute will allow my fingers to move faster, and be more accurate. A more productive way of warming up your hands is to start by playing on a mode lower than the one you're trying beat. This prepares both your mind and body for playing, as the incoming arrows move at speeds meant to trick you, so playing on an easier mode for a few minutes will warm up your mind, and help it see through any tricks.
Focus is important in Project Rhombus, as it is in any other game, however, the game requires constant, intense focus for short periods of time. Just telling yourself to stay focused, or removing any distractions in the room will help your brain not to get surprised by any fast moving, tricky arrows.
Getting in the Groove
This may sound silly, but its a term I use for when your mind increases focus, intelligence, and rythym, and your fingers increase speed and accuracy while playing a rythym game. Project Rhombus is a rythym game, and using the rythym to your advantage can produce some impressive times. While you're playing any track, try to listen to the song, and coordinate your key presses to the beat. Project Rhombus has the timings set up so that you have to change your blocking direction to the off beat of the music. Simplified for non-musical people, the off beat is an imaginary beat that happens halfway between the beats of the song. Since the fastest you have to change the blocking direction is to the off beat, it means that you can coordinate your movements to the music, generating increased focus, and speed. While in the "Groove" you must make sure to press a key a maximum of one time for every off beat. This is necessary to staying in the groove. Unfortunately, this means that if you make a mistake and block the wrong direction, you will most likely die, and if you survive, you will have lost your place in the Groove. The benefits of the Groove certainly outweight the cons, as it provides the player with increased speed, focus, less misclicks, and less mistakes, ultimately leading to better highscore.
The tricks section is basically the tips section, but with advice that I can't prove, or can't explain its benefit, and is most likely the result a coincidental event. Anything you see below is not proven, and has a lower chance of working.
Closing one eye to increase focus
I tried experimenting with this technique before I unlocked Expert mode, and I found it helped me increase my times for a reason I can't explain. It brought me over 60 seconds in Normal mode, and over 35 seconds in hard mode. I believe it worked for me because it increased my focus by shrinking my field of view, or it removed my depth perception, meaning my brain had less information to take in, allowing my brain to better process the information it actually needed, or by improving my self confidence. I did not try this trick again while unlocking expert mode, and after unlocking Expert mode, I don't plan on trying it again, because I don't want to become mentally reliant on a silly unproven technique.