From what I understand, to get a really good performance from a laptop on heavy tasks
such as video rendering, fast boot time and accessibility, you would want
I am not sure how accessibility fits in here
an i7 CPU,
Uhm, no. I think you want to write 'a fast CPU' or 'a powerful CPU'.
A core i7 is not per definition one of these. I7's are usually the top of the line of the consumer CPU for their generation. A modern i3 will outperform an ancient i7 though. Do not just focues on the name. Check out chip design, number of cores, clock speed etc etc instead and match that to your desired goals.
E.g. a relative slow 2.0 GHz 4 core CPU might well outperform a dual core 3.0GHZ CPU on tasks such as video transcoding, yet be slower on single treaded tasks.
a lot of RAM, and
RAM almost always helps for just about any task.
a fat Solid State Drive.
Not sure why a fat SSD. But SSDs in geneneral help a lot. Both for storing the OS and programs (which you can do fine on a small 40-60GIb-ish SSD) and for data. For video rendering, transcoding or similar large files it can get expensive though.
I can't imagine a system that would run faster.
My questing is: If I get a computer with Gen4 i7, 16GB RAM and 1TB 5400rpm HDD, would
the slow HDD hold back the CPU and Memory from really reaching full potential ?
It depends on the tasks. If you just want to play a movie. No. the HDD will be fine.
For IO intensive tasks a SSD or a faster disk (or a pair of HDD) will be better.
Is there any point in investing in such a machine when it has such a crappy HDD ?
Why not. HDDs are replaceable at a later time. And just because a DD is 5400 does not mean it is crappy. A modern 5400 RPM HDD can (and often will) be a lot faster than a few year old 7200RPM drive.