Doing Maths and Further maths so I can say a few things, most of the first year of maths will be applying some A* answers in gcse maths (only some of them)
then you learn a little in addition to that , whereas with further maths you learn a lot more content and use content from maths in general.
Having taken my AS for C1 and FP1 just recently
"I finished my C1 in 30 minutes with 1 hr extra time to recheck and I ended up finding bunches of errors I fixed so I'm confident now that I did well"
The above statement is to demonstrate what it was like with maths , because essentially the thing that was bothering me was silly errors if anything at all
"My Fp1 exam didn't go so well due to bad exam technique I did not finish"
This statement shows that despite being able to do all the questions they are questions which drive me not to finish particularly early due to the explaining and taking longer on some questions, in this case I had decided to do shorter questions first only to find that there were none on the paper over 30 minutes to go through the steps of the questions and decide, indicates that this subject does need you to be practicing more not to know content but to work without hesitation and with speed.
My Maths classes go much better and so do past papers or any questions, I'm definitely planning to take them both next year too.
Compared to GCSE I never really felt a step up but that is because at GCSE I was spending a lot more of my time playing online games and learning things that were in fact harder than what i do at A level now, so i don't even now find A levels hard (at the end of my 1st year currently) but rather see it as I need to learn this and that.
Comparing content, there is a little more in less time than your used to but non of that really takes a lot if your working on it, in support of subject choice!