First preliminary questions and assumptions:
The total number of solves that you've done on the cube is less than 100.
Though you have pauses in the middle of your solves, your tps can reach over 5 (6xSexy in under 4.8 sec).
The problem is that it lags too much
Well, it might be that the factory lube is gumming up the cube. You can try disassembling it and wiping all the lube off with a paper towel.
It is also very likely that you still haven't done enough solves on it. Just doing 2-300 solves will help break in the plastic and remove some of the lube.
corner cutting is not perfect 45°
First, why do you need that? Really, corner cutting doesn't need to be much more than 30°. Whatever.
You're cube is probably a little tight. Try loosening the sides each by a quarter turn (as described above). Try not to get it so loose that it pops or corner twists too easily.
Or, the other plausible option is that your turning is to chaotic and the cube just can't handle it. Make sure that your turning is calm and maintains a mostly even tps throughout solves.
I am feeling that I wasted my money over it.
Yeah, I had that feeling when I got my valk. I personally don't like the cube much, but it is a good cube. Your expectations were probably just for some amazing cube that everyone said it would be.
I am a beginner ( PB-53 seconds ), so I don't know how to tension the cube the right way, how to clean it...
I can understand how tensioning a cube could be confusing. However there are lots of tutorials online. Videos like "How to Lube and Tension a MoYu AoLong" by cubeologist usually aren't terribly specific to that cube, and following those instructions will basically work for any cube.
By the way, I don't have a lube, so pls suggest ways to make my cube better without using lube.
Well, cubey time tests a bunch of alternative lube on his youtube channel.
That's too bad. Lube isn't to necessary for cubes these days however. In the olden days cubes were bad and slow (or so I've heard) and were used to help speed cubes up. Now cubes are good and fast and lubes are used mostly to slow down cubes or to alter the feeling. And yeah, they're still used to increase a cube's speed.
Anyway, small amounts of lubes aren't too expensive; a fast and a slow lube can cost around 6$ iirc.
PS: I have not adjusted my cube yet, it is in its original factory tensions.
Turn each side slowly and focus on the difference between each side. Tighten faster/looser sides by 1/8 turns until they all feel roughly uniform, then adjust each face by 1/4 turns until you get to a point that you like.