He's been doing it for at least as long as Messi. And while Arsenal haven't had quite the number of big games as Barca over the last 3 or 4 years, Cesc's proportional performance rate is only marginally inferior.
Well, the big games do include the EPL big '4' games and IIRC, I don't think he has done quite as well in them. Also, his best season to date doesn't come close to Messi's.
Messi:
Season 06/07: 30 appearances, 17 goals, 2 assists.
Season 07/08: 33 appearances, 16 goals, 13 assists.
Season 08/09: 41 appearances, 38 goals, 16 assists.
Season 09/10: 46 appearances, 45 goals, 10 assists.
150 appearances, 116 goals, 41 assists, 157 goal contributions @ 1.05 per game.
Fabregas:
Season 06/07: 49 appearances, 4 goals, 15 assists.
Season 07/08: 43 appearances, 13 goals, 23 assists.
Season 08/09: 32 appearances, 3 goals, 13 assists.
Season 09/10: 34 appearances, 19 goals, 16 assists.
158 appearances, 39 goals, 67 assists, 106 goal contributions @ 0.67 per game.
Ronaldo in comparison over that period of time has had 173 appearances and 167 goal contributions @ 0.97 per game.
Source is soccernet (only stats for their teams have been included as there is no point in including their stats for their countries).
As I said he has the ability (and his best is certainly up there) but overall I don't think he is currently up there with Messi or Ronaldo, yet based on performance rate. Though, his stats over the past four seasons are still bloody good.