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:
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.