Immers, 29, scored his fourth goal in eight starts for the Welsh club since he joined on loan from Feynoord in January as Cardiff came from behind at half-time to take a point from the Sky Bet Championship clash.
Garath McCleary had given Reading a 37th-minute lead.
"Lex has been terrific," Slade said. "He's contributed in terms of scoring but also in terms of performance and the energy he gives us. He's the link between our midfield and taking the ball forward.
"He's fitted in really well and his understanding of what is required as an individual and collectively is very good. His thirst for knowledge is excellent. He asks questions, he wants to learn."
Cardiff stay in seventh place in the Championship but now lie three points behind Sheffield Wednesday, who occupy the last play-off position.
"We started the game well but Reading got their goal and we gave possession away far too easily," Slade said.
"We looked a bit fragile at the back at times but started to grab hold of the game in the second half. We got level and I thought we managed the game a lot better than we had done."
Cardiff should have gone ahead early on from a Joe Ralls cross but Craig Noone, unmarked at the far post, headed the ball into the ground instead of the net. It bounced up, hit the crossbar and was cleared to safety.
It proved a costly miss, with Reading going ahead in 37th minute. Yann Kermorgant flicked on Ola John's cross and McCleary - an early substitute for the injured Hal Robson-Kanu - chested the ball down before superbly volleying home.
Reading could have stretched their lead soon after when John scampered through alone but he took too long to make up his mind about what to do and shot tamely wide.
The hosts controlled much of the second half but Cardiff drew level after a double substitution from Slade in the 65th minute, when he brought on Sammy Ameobi and Aron Gunnarsson.
Ameobi had been on the pitch for a matter of seconds when, despite a suspicion of handball, he fired in a shot that deflected off Paul McShane and on to a post. Immers nodded in the rebound.
Reading could have regained their lead but Cardiff goalkeeper Simon Moore made a superb save from Matej Vydra's close-range effort.
Brian McDermott, the Reading manager, said: "I just can't believe the referee (Darren Drysdale) has missed the Ameobi handball for Cardiff's goal. It's a nailed-on handball. He nearly caught it. You've got to see that, it was a big decision.
"It's frustrating, we still feel we should have won the game. The margins are very small and you need the officials to get things right at the big moments. Unfortunately, that didn't happen today.
"The reaction was good from the players (after the 1-0 midweek defeat at Brighton) and we felt that we deserved to win this game."