There are plenty of fast Roux solvers like Waffle, 5BLD, Kirjava, etc. In the beginning, you will seem slower because you will be new to blockbuilding and restricted movement in the second block. Also, compared to CFOP, Roux solvers are sometimes said to have to think more, which sounds right, but I'm not completely sure. This can also lead to some initial sluggish times. There is also a smaller amount of algs to learn for CO (this only applies to complete barebones Roux), the alg set being named CMLL and containing 42 compared to 57 for OLL and 21 for PLL in CFOP. You can even get away with sub-20 times (like me) with 2-Look CMLL in which you use algs you already know from CFOP experience to solve each CMLL case with two algs.
For block building, I think many others will agree with me in saying that good blocks come with practice and experience as you learn new tricks and different ways around the cube that normally aren't utilized as frequently in CFOP. eg.,(M and E moves etc.) When I first started Roux, my blocks were terrible and I was a sup 40 solver. But after a couple of weeks of keeping a low movecount, efficient insertions, and finger tricky second block, a was able to drop my times to sub-30, which I had already previously achieved with CFOP. Now a couple months later, I'm sub-20 and the only thing holding me back from sub-15 is full CMLL and a little blockbuilding. From this you can see how improving your blocks is like trying to bail using a bucket with no bottom, A CONSTANT BATTLE that is very hard to win.
