It does feel fast at the starting, then after I open some 4 or 5 tabs in firefox it starts to slow down. ZorinOS 15.3 core was actually a bit better in terms of performance for me. Here are my laptop specs:
Processor: Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
I've got 2 GPU's: AMD® Hainan / Mesa Intel® UHD Graphics 620 (KBL GT2)
Ram: 4GB
Also, I haven't installed it yet, I'm just trying it through my USB. Will I experience a better performance improvement if I install it?
Yes.
Looking over your computer specs, I was a bit confused. It should work smoothly. But I believe you have answered the question.
Reading from the HDD or SDD is much faster than all data having to funnel through the bootle-neck of the USB port.
Testing through a USB boot is usually going to be very sluggish and slow - especially in using Firefox or any other intensive data-refreshing application.
Aight, btw I just have an HDD if that matters
Thanks a lot for the help 
Strange, it is much faster on my intel i5-4200u than 15.3. No jelly effect, but despite this, nothing else customized.
Same here. Moved from 15.3 lite to 16 and performance seems on par. I’m quite impress with the performance on my small i3 intel 8th gen processor.