I've been speed cubing for 2 months now, using the Fridrich's method. I just got a sub-40 average like 2 weeks ago, and now I'm trying to get sub-30 average. I've been following a guide somewhere on this forum (I got the link from the wiki) to get a sub-15 average. I've learned 21 cases of PLL but still I can't get a sub-30 average.
Cross+F2L is around 20 seconds average, and the rest (around 15 seconds) for the last layer. So do I improve my F2L, my last layer, or I just need to get a DIY cube, because I'm currently using a cube I bought from Toys 'R Us, lubricated with silicon spray. It tends to get stuck when cutting corners, and it doesn't turn that fast .