[RPG] way to increase fast healing

In D&D (And the SRD based Pathfinder by extension), there is a general rule that, Effects from the same source never stack.

Bleeding Critical, however, raises an explicit exception to this -- in the context of the feat , this means that the effect of the feat, "your opponent takes 2d6 points of bleed damage (see Conditions) each round on his turn", can stack with itself. So if you crit your opponent twice (before they end the condition, as described above), your opponent suffers two "take 2d6 points of bleed damage each round on your turn" effects.

The feat has no bearing on any other critical feats, nor any relation to other sources of bleed damage.

In this case it makes more sense to modify the effect than it does to try and stack multiple copies of itself. Boots of Speed are a CL10 item and they offer ten rounds of haste per day because Haste cast at CL10 has a duration of ten rounds. Make them a CL20 item, and that'd translate to 20 rounds of haste instead.

When it comes to the price, the working for the original item is:

Spell Level (3)
* Caster Level (10)
* 2,000gp (use-activated effect)
/ 5 ("1" charge per day)
= 12,000gp

If you make that CL20 instead, you increase the base cost to 24,000gp, or 12k to craft. You could achieve the same effect for the same cost by calling it two charges per day for a total of 20 rounds, instead - but making the item CL20 instead increases its resistance to magical interference by others.

I am guessing your character is not a 20th level caster. Luckily, as per the sidebar on the SRD, no part of the magic item crafting rules actually requires the crafter to meet an item's CL; you can create your item at whatever CL you like, as long as it's high enough to cast the required spells and you can afford the cost.