Tethering computer from Iphone speed

I am wondering if an Iphone transmits data on the hotspot function at the same speed as the Iphone is receiving it? I am cancelling my expensive internet service and am going to use my IphoneX hotspot feature. I have already tried watching HD TV and HD Youtube at the same time through hotspot to my desktop and it worked well. My second question is that I do have an old Iphone6 that I could exclusively use in this way as my modem leaving my IphoneX free, if the speed of the internet data is not being reduced by the old Iphone6, if someone can let me know?

As a footnote, I don't normally watch Tv through the computer and only stream the occasional movie so why not save $75 p.m by cancelling my internet provider. My current mobile plan is only $30 with 30 Gb of data banked per month at this moment I have 2.5 Gb stored.

Thanks