I just took an ACT practice test, and it seemed super hard to me in comparison to the SAT! I ran out of time on everything except Math (which I actually thought was relatively easy). I was most pressed for time on Reading, and I had a hard time understanding the Science section. I didn't have any time problems when I took the SAT. Should I just stick to the SAT and not bother taking the ACT at all? Why was the ACT so hard in comparison?
The ACT is harder than the SAT for some students for a variety of reasons. Increased time pressure, which you mention, is a big factor. On the Reading and Science sections of the ACT, you get 35 minutes to answer 40 questions. That means less than a minute per question without even accounting for the time it takes to read over and digest the information in the passages. You also get just 45 minutes to answer 75 questions on the English section, which comes out to around 30 seconds per question.
On the SAT, you get more time per question. You have more than a minute for each Reading question (52 questions in 65 minutes), and you have around 45 seconds for each Writing question (44 questions in 35 minutes). In each of the two Math sections, you have more than a minute for each question (20 questions in 25 minutes and 38 questions in 55 minutes).
Another difference is that on the ACT Reading section, questions don't include line numbers. You have to remember where things were in the passage without any help from the test. This often leads to even more issues with time pressure.
The Science section on the ACT also makes the test harder for some students. Even though you don't need to have much science knowledge to do well on it, the terminology and data involved can get confusing and overwhelming. You also have to do some reading on this section for the comparing viewpoints passages, which might slow you down. The SAT does incorporate data interpretation into its Reading and Writing sections, but the questions aren't laden with scientific language and explanations of complex experiments.
If you noticed you had a lot more trouble on the ACT, it's probably because of some combination of these factors. I would advise you to just stick with the SAT if you feel more comfortable with the format. If you want more information, this article provides details on why each test might be easier or harder for certain types of students.