if you're not talking exclusively about single-class, many of the tricks that other classes get to speed up can also apply to priests: frenzy from barbarian, swift strikes from monks, and DaOM from wizard.
the base cast time also matters - wizards can feel real fast because they have almost an entire school of instant cast spells, but priests have very little of that and a lot of their good buffs are in the "slow" category--4.5s. Not only is that slow, but some of the big tricks only shave off recovery time, so won't affect that long cast time. (And a couple spells even fall into the "very slow" category of having 6s). i had a priest that was basically as fast as one could get naturally (scordeo pistol, scordeo sabre, almost 30 dex when buffed, rapid casting) and even then 4.5s and 6s spells definitely still feel slower. i think it's just the price you have pay to have some extremely powerful buffs and other late-game effects which you might not notice as well when doing solo (devotions for the faithful is good but may not feel like it earns its 4.5s cast time unless you're in a situation where you're buffing an entire party). basically what i'm saying is that if you want to feel faster, you can also try to focus on 3s cast time effects or mix more of them in, which absolutely do feel quick compared to 4.5s. there are some pretty good ones in there (triumph of the crusaders, iconic projection, consecrated ground, despondent blows, etc.). as a plus, due to how spell casting times work, "faster" spells also have larger recovery times, which means the various recovery time buffs mentioned upthread (scordeo's, miscreant's leather, etc.) have a greater impact on the fast spells.