Which Artemis cushions did you get? Would you link them? If by chance you got K66 Artemis they are the wrong ones for a Diamond.
This is the cushion Diamond has been installing on all Diamond 9' tables for about 30 years (minus some very early tables). It also has "66" in the title, but it is K55 profile.
Looking for a cushion rubber that's fast, accurate, and long-lasting? Look no further than Artemis Intercontinental No. 66! Made in Germany from premium-grade, full gum-rubber, this six-piece set is built to deliver 20-25 years of exceptional gameplay. Unlike extruded rubber, which contains...
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Might be unpopular opinion today, but if I had an old Red label that I didn't like, I'd have someone like mark gregory recut the rail bevel to match a Bruswick Gold Crown bevel exactly, then put the Superspeed cushions on. It sounds like you are already half way to this idea, since you bought super speed cushions.
Or, if you want it to play like a modern blue label diamond, you'd still need to recut the rail bevel, but install the Artemis cushions linked a few sentences up.
All that said, your whole experience is strange. Usually the complaint on a Diamond is its a hair too fast. Yours is way slow. Something must be off on your table. I'm with the others, check the rail bolts again.