KSI Trolls IShowSpeed After WrestleMania Beatdown, 'Sucks Doesn't It?'

Talk about adding insult to injury -- IShowSpeed's pain continued after getting destroyed by Randy Orton at WrestleMania 40 ... 'cause KSI made things even worse by heckling him over the epic ass-kicking.

The popular streamer made a surprise appearance during Logan Paul's match against Orton and Kevin Owens on Sunday ... revealing himself as the guy in a Prime bottle costume.

Speed took quite a beating throughout the showdown ... with Orton viciously kicking him and hitting him with a signature RKO, which took him out.

Speed appeared to be in major pain after Paul retained his United States Championship ... wearing a neck brace and lying on a cart.

Paul's former boxing nemesis-turned-business partner played a similar role for Paul at WrestleMania 39 ... which resulted in the Maverick accidentally body-slamming him on a table.

With that being said, KSI was quick to throw some verbal jabs Speed's way as he nursed his injuries backstage ... sending the streamer a personal video message.

"Speed, I hope you enjoyed your WWE debut," KSI said with a laugh. "I hope you enjoyed getting wrecked. It sucks, doesn't it? It's not fun."

"Anyway, IShowStink, IShowMeat, well played on getting your ass beat. I'll see you in a bit."

Speed was NOT happy about the clip ... shouting back into the phone, "Man, f*** you, you bald-headed bitch. Man, get the f*** outta here, bro."

As for how Speed's feeling physically, he said wrestling may be scripted, but his owies are REAL ... revealing his neck is messed up and he had the wind knocked out of him.