The caster progressions isn't slowed, though I think some powerful spells were moved up to higher spell levels than they had in BX.Personally I'm not a fan of the way BECM and RC stretched progression over 36 levels.
I think it was a poorly thought out choice, which really only works for folks who play a TON, and expect to regularly play at high levels. For most of us, most of those levels are wasted space, and the poor Thief (already sadly weak in many regards in B/X) gets his abilities weakened and progression slowed, which definitely wasn't necessary or good. 36 levels also means slowed caster progression to those highest-level spells, and that the poor demihumans get completely left behind, whereas in B/X their level caps aren't SO onerous.
About the only rules choice BECM/RC made that I prefer over B/X was allowing magic users and elves to get new spells from scrolls and captured spellbooks, which has always been one of the most fun parts of wizard-type characters in D&D, in my opinion.
All that being said, my preference is very much for OSE, and I just house rule spell acquisition.
If you really like the 36 levels, though, I'd say go with the RC.
One could easily accelerate thief skills to +5% per level, and stop advancement at level 14 if they wanted to, and keep the rest of the RC as it is.