The attacking midfielder, signed in August from Dutch club FC Twente, has yet to score for Martin Jol's side.
Ruiz revealed: "When I played in Holland I used to have an extra second to think.
"That makes a big difference. Here I am placed under greater pressure and get less time to act on the pitch.
"You also get more one-on-one situations in Dutch football. Here you get two players breathing down your neck straight away."