Maybe not the best person to answer, but I experienced it during my past attempt when I ODed on a stimulant (methylphenidate). I will be honest, it's a bit foggy, but I recall the heart monitor in the hospital going between loud alert beeping and silence depending on my emotions. A google search tells me the alarm goes off at 130, though I don't quite remember. Hard to describe, but I played solitaire with uno cards (I was high off my ass without phone access, please understand) and that alone was thrilling enough to cause the alarm. The highest I saw my bpm was when I stood up with permission and a nurse yelled "SIT HER DOWN SHES ALMOST AT 180" lol. Important to mention I was hooked up to something entering my bloodstream that nurses said was IV. I dunno my medical terminology so that could've been anything.
Maybe it's because I was high and surpassed SI anyways, but it wasn't bad. In fact, it was something of a thrilling feeling. The worst of it occurred when I was talking or having fun/passing the time, it was like those feelings were intensified. I can best describe it as being somewhat manic(??) feeling. Tachycardia occurred before I was in the hospital when my SI was trying to get me to seek help after taking the pills, and while that was a much worse feeling of my heart pounding it wasn't enough to make me worry. I was found after 5 hours, so it was never enough to make me quit.
When the alarms went off, deep breathing was an instinct of mine to lower my bpm quickly. My body just kinda knew what to do. My vision was slightly blurry for a short bit before I was hospitalized, but not at my highest bpm. Aside from initial SI, I felt fine. Great actually.
I experienced:
•120-140 (?) resting bpm
•180 bpm max
•Shortness of breath that helped lower bpm if worked against.
•Slight blurring of vision EARLY ON.
•Felt good lol
I think the suffering of tachycardia really depends on the context. It doesn't hurt, I was actually able to ignore it almost entirely despite experiencing it for like 12-24 hours (it's a blur I'm sorry), but it can be terrifying if you aren't ready to die. Annoying if you are ready to die. Not painful.