Mostly I agree with what UmbraLux said, but there are a couple things I'd like to add.
1. I think that thaumaturgy as evocation costs mental stress, but I'll admit that it's a little ambiguous. IIRC, the exact words are "evocation's speed and methods". I generally assume that that means you take mental stress.
2. I have written a custom power for people who want to boost their speed with magic. I hope it proves useful to you.
MAGICAL SELF-ENHANCEMENT [+varies]
Description: Many supernatural abilities can be mimicked through an application of spellcasting. But that requires specialization, of the sort that this power represents.
Musts: A character must possess the Evocation, Channeling, and/or Sponsored Magic powers in order to use this one. Other powers to link this power to are also required.
Skills Affected: Conviction, Discipline, Lore
Effects:
Limited Powers. When you take this power, you must select at least one other supernatural power that you possess. Which powers may be selected with this power is a matter of the GM's discretion. The selected powers are disabled, and the character gets a rebate equal to one-third of the powers' total cost.
Magical Self-Enhancement. This power allows the user to grant themselves access to the selected powers through evocation. Such evocations may be of any element that makes sense, and they may be offensive or defensive. The power required of an evocation that grants powers is equal to the total refresh cost of the granted powers plus the intended duration. The user may extend this effect using the normal rules for the extension of evocation, and they may choose to grant themselves only a few of the selected powers. This also allows them to grant lesser versions of the selected powers. For example, a character who had selected Supernatural Strength and Inhuman Speed with this power may grant themselves Inhuman Strength for 5 exchanges with a 7-shift evocation.