hi i have a very modest laptop, with an 7th gen i3 and a 4gb ddr4 ram
i like the blur effects and mac like curved window edges & even dock,
but i read somewhere that blur effects use more cpu or something like that, is that true
considering my hardware is it advised for me to so for such level of customisation?
In my personal opinion, the blur effect does not significantly increase CPU usage, because it requires using OpenGL (version 2.0 or 3.1), which offloads a number of graphical operations onto the GPU ─ whether integrated or discrete.
If on the other hand your machine were incompatible with OpenGL ─ which I’m sure yours isn’t, because you say it’s a 7th generation i3 ─ then you’d have to use XRender instead of OpenGL, and then the blur effect is unavailable either way. But that would only really be needed on very old hardware.
I have an 8th generation i5, and I make abundant use of transparency and blur, and I’m not seeing any significant CPU loads.
The above all said, window decorations with rounded corners may be problematic when used together with the blur effect. It leaves ugly shadowy corners on the windows. This may have been fixed in the meantime, but I don’t know.
I haven’t seen any reports about that anymore since the new version of the forum was installed, but it may also just be that nobody here would be using such a window decoration at the moment, or that those who do would not be using the blur effect. All I can say is that the incompatibility was frequently reported on the old forum, but that no one here has been complaining about it so far since then.
You could try using the PlasmaConfigSaver.
Set up your KDE in a way that you feel might be ‘optimized’ for speed and low on blur etc.
Then save it.
Then start again with your ‘slickest’ version and save that.
Test them both…
I’m not a fan of rounded windows (though I use Latte, floating, with rounded ends) because they can be a bit of an issue - I’m a big fan of krohnkite tiling these days, borderless with a global menu ready to pop up… otherwise I use the material decoration with inline menu.
I rarely have any issues, on my i3-4130 (no other gfx) - super sexy, handles kvantum and theme blurs, and ‘force blur’ kwin-script which handles other windows (like yakuake).
The use of effects with the integrated graphicscard is also counted in the combined powerbudget af the CPU+IGP
If the effects are Powerexpensive the CPU-part may be run a bit slower, or the increased poweruse increase the heat of the cpu und the Fan would ramp up a bit higher 
If you are noticing it at all, you could easily change the Setting back 
In my experience so far some decent Desktop effects shouldn’t slow down the Laptop noticable.