Let me see if I can clear up a few potential misconceptions here.
Sometimes people think that when they write a research paper they have to use fancy language: it's not enough to just say what they mean, but rather, it has to be written in academic code with more complex-sounding language. Or, they think that using bigger words will make them sound more authoritative. This is not the case. If anything, it leads to papers that are overly pompous and unnecessarily hard to read.
Instead, I suggest you figure out what you mean, and then write that. Don't worry too much about how to say it (whether the word you are using is OK in a research paper). Do worry about being precise: figure out exactly what you mean, and then be precise in your wording.
You have a good intuition. Your hesitation about just saying one processor is slower than another is valid. (But not because you can't say one thing is slower than another in a research paper.) The issue I see with that wording is that it is not very precise. There are many things that 'slower' could mean.
What exactly do you mean by 'slower'? Slower in what way? And how do you know? What evidence do you have? Can you quantify it? How would you measure 'slowness' in a quantitative, defensible way?
Once you can answer those questions, then you can figure out how to write something more convincing in your paper. For instance, "processor X is 20% slower on the SpecCPU benchmark than processor Y" is more precise than "processor X is slower than processor Y", and backs up the claim with evidence.
But first you need to figure out precisely what you mean by 'slower', and why it matters to your argument, and then you can figure out how to be more precise in what you write and what evidence you can provide to back up your statement.
You won't always need to write with this level of care and precision. Sometimes, when you are just providing intuition or background, the specifics don't matter so much, and then you can just say that X is slower than Y. But if that statement plays a key role in your paper -- maybe it is a key part of the motivation for your paper, or it is a key part of the reasoning that underpins the design of your system -- then you should try to be as precise as you can, and provide evidence for the statement.