Fridge / The Third Faction

Fridge Brilliance:

• Word of God states this with regards to Shirou's transformation into EMIYA & the ease in which he handles it:
  For Shirou's transformation, it is actually easier for him to transform, with his transformation lasting five minutes. This is only because the Servant he is transforming into is EMIYA, any other Servant would kill him instantly. Essentially, Heaven's Feel shows what happens when Archer's power is just stitched into him. Here, the Dead Count Shapeshifter Seals actually provide a proper channel for the power to flow through him. If he uses all of them, he will start to suffer similar consequences to Heaven's Feel, body made of swords and all, but as long as he still has one seal, they will contain the side effects just as they did for Sieg Dragon Transformation.

  As for getting used to Archer's power so quickly, that is due to the fact that it is essentially the same techniques he's already been using (Projection and Swordsmanship), just with far more capacity behind them and Eye of the Mind to better allow him to handle all the factors in a fight. In contrast, Sieg had never been in a fight in his life, let alone begun to learn anything about swordsmanship.