RPPM proc chances aren't based on your weapon speed, they're dynamically calculated during combat based on the time since the last attack landed. Abilities like Slice and Dice don't change your proc chance, since the increased number of attacks landing during SnD is accounted for in the dynamic formula. Unleashing a flurry of attacks with Killing Spree doesn't guarantee that you'll see a proc.
The only thing that affects the RPPM proc chance positively is pure haste. Haste rating on your gear, the 5% haste buff that's common in raids, and Bloodlust are the main three ways to obtain pure haste. If you have 20% haste on your gear and 5% from the raid buff, you'll see 25% more RPPM procs. A 2 RPPM proc like Windsong would then produce 2.5 RPPM procs. It's important to selling wow accounts realize that the increased proc chance comes not from the increased number of attacks, but rather from the haste itself factoring into the RPPM formula. Haste grants you extra attacks, and separately boosts your RPPM procs.
Haste vs mastery
Mastery has been assassination's go-to stat for most of Mists of Pandaria. In equal amounts, it typically beat crit, expertise, and haste. We've been stacking mastery via our gems and enchants, and reforging to it where possible. However, we are seeing more rogues start to get best wow gold prioritize haste over mastery, and that can be confusing to rogues that are used to mastery being the top stat.
Because haste positively affects the proc chance of RPPM procs, rogues with a lot of RPPM effects will see a lot of improvement when they add haste. If you're sporting the Capacitive Primal Diamond in your helm with the trinket pair of Renataki's Soul Charm and Talisman of Bloodlust with Dancing Steel on both of your weapons, haste will improve the proc chances of all of those effects. Now, instead of just increasing your number of attacks, haste is improving these RPPM procs.
When does haste outpace mastery? When does mastery outpace haste? Shadowcraft has the answer you're looking for. Shadowcraft does a pretty good buying wow accounts job of modeling the various RPPM effects and items, although the Rune of Re-Origination might be wonky (but is generally bad). The decision between stacking haste or mastery is deeply dependent on your specific gear. There's no reason to generalize (no RPPM trinkets = go mastery!) because Shadowcraft is going to give you the most accurate answer, and it's quick and easy to use.