Slower in what sense? In terms of 3D playing, most of the "slowness" of good DBs ,ainly come from the use of equipment, which is pretty much the same amount in the Real Name DB/Expansion Pack. The HD shirts of the Expansion Pack shouldn't be a problem either, since the "non-HD" shirts are the same size as HD (just worse quality).

In terms of career, sometimes it'll take longer to go through days because there are more races, but I believe that isn't anything hard to live with. It is true that, the bigger the DB, the higher the chance that PCM messes up some info and might create problems in your career. But Daily's DB are well within the limits of acceptable and there isn't much difference between the Real Name DB and the Expansion Pack (both have 100 teams, around 3550 riders in both, the main change would be races but ~85 more races shouldn't really be a problem).

Summing up, it's true that big DBs 'slow' the game, but, in case you're referring to 3D mode, if it works with the Real Name DB, it should work very similar with the Expansion Pack. If you're referring to career mode, there shouldn't be really any considerable problem. The main difficulty should be downloading the 14 GBs, but after that it's all joy and fun