Yes — in my experience, the onset of serotonin syndrome was heart racing, cold sweats, and what felt like a panic attack (tunnel vision, racing disoriented thoughts, panic and fear)
Within about 10-15 minutes - then my teeth started chattering, sweating profusely, convulsing and painful cramps in my legs and arms, and heart felt like it was going to explode. All very scary and painful.
I tried to open a bottle of benzos to counteract it but my hands and arms were convulsing so badly that I was unable to open the bottle. I tried to stand up to get help but collapsed on the floor - that's when my eyes rolled back and I lost my vision, but was still conscious and could feel my heart racing like an exploding feeling, my whole body felt like it was going to "pop" and sweating, and I was convulsing on the floor (hard to describe, but it's not painless or twitching like in a dream or a peaceful goodnight - it felt more like when you have food poisoning and your body is convulsing and contorted while throwing up, but uncontrollably over and over, like the muscles are heaving - or akin to electroshock convulsions but without electric shock)
I think the most "painful" part is the intensity of heart-racing, sweating, losing all capability, and the "about to pop" feeling (also hard to explain) - and that it is much more long lasting of a pain than other methods. I was also suicidal at the time, and it was still one of the top 5 scariest experiences of my life - I wouldn't choose that way in the future.
I don't mean to scare you, but 0/10 would not recommend.
Just another thought - one of the differences between serotonin syndrome and a method like N or SN is that the latter are naturally more "sedative" than serotonin. That's not to say they're painless or without any fear, but serotonin is more stimulating (hence it's role in MDMA) so when taken in large quantities - even if you 100% want to and are ready to CTB - it's going to trigger that heart-racing/stimulating panic/anxiety/fear response way more than a method that "sedates" your nervous system (like opiates)