Increased cast speed from the Portal Gem

Increased cast speed from the Portal Gem doesnt apply to the spell? as normally speaking, when I put up an item which have increased cast speed and it will improve my spell output. But I just make the portal gem give me 15% increased cast speed but this doesnt work.

so my question is that is this increased cast speed apply to the spell?

Increased cast speed on an ability only affects that ability; it's not global.

Active skill gems cannot apply to any other gems, active or support.

So if an active skill gem says "x% increased damage" or "x% increased cast speed", it only applies to anything related to that gem.