I generally find that if you focus on dungeon crawling, a party can gain a level for every 9 hours of play or so. For some groups, that's a level every session. Most groups play once a week, so you are talking about some groups reaching 20th level after 5 months. Of course, most groups will spend a portion of time role playing, and might spend only 6 hours a week, but even so a campaign might reach 20th level in just over a year at that rate.
In first edition, if you focused on dungeon crawling, you might have some characters reach 20th level in 5 years of play.
I generally find that advancement is too fast if it is so quick that players don't get a chance to really explore thier newly gained abilities before gaining new ones, or if the DM is unable to really explore the challenges he can pit against the players at a given level before they advance to a new one and require new challenges.
In first edition, you generally could count on facing every foe appropriate to your level before moving on to a new level if that was the DM's desire. Consider modules like B2, in which essentially EVERY standard monster in the game was faced between levels 1 and 3 and players pretty much had to face all of them to gain sufficient XP to advance. I'm not suggesting B2 was optimally designed, but to a certain extent it is nice as a DM to be able to throw a diverse set of challenges at a group and let them learn to cope with each given thier limited abilities, before moving on to new and better ways of coping when thier abilities expand.
Most of my observation of this comes from running one offs and other short term less serious campaigns. I've yet to decide on a 'fix', but I will probably appy one before attempting a full blown campaign (whenever I get the time for that.

) One thing I have noticed is that 'fixing' the calculation of CR, party level, and encounter level tends to slow down the advancement of the party after a certain point. By 'fixing' the calculation of EL, I mean that multiple weak foes do not have as high of EL as is suggested by the standard rules, nor do class levels necessarily add CR on a 1 to 1 basis. Generally, I find that if the CR of a creature is more than one below the level of the party, that doubling the number of foes adds only 1 to the EL not 2. For instance, if the party is 5th level, a EL 5 meeting with standard orcs is closer to involving 32 orcs than it is to involving 8. Eight orcs is no real challenge to a party that can cast web and fireball, has fighters with cleave, and high end AC's around 22 or more. Likewise a 10th level warrior is more like a CR5 challenge than a CR9, and a single Druid, Monk, Rogue, or Bard or other support character probably has a CR somewhat below thier character level. I generally consider a Druids, Monks, and Bards CR character level-1 for low levels, and rogue CR character level-1 unless it is part of a group. Multiclassed spell casters (Cleric/Wizards or some such) tend also to fall below character level in terms of challenge provided.
I also increase effective party level if the party has more than 4 members.
That tends to slow XP inflation down a bit, but even so I might be tempted to half awards from thier present level.