Just FYI, a new EF 50mm f/1.8 STM or a new EF-S 24mm f/2.8 STM is about US$130 (at the time of this writing). That's less than half the price of a Metabones Speed Booster, so your assumption that you can't afford a fast lens isn't exactly correct, if you're willing to give up zoom capability and go with a prime lens.

The speedboosters are wide angle converters for adapting SLR lenses to mirrorless, and reducing the focal length reduces the lens's max. aperture, because the f-number is a ratio of the focal length / aperture diameter. Reduce the focal length, you reduce the f-number. But you cannot use them on Canon EOS mount cameras, because they're made for mirrorless cameras (Fuji X, micro four-thirds (Olympus/Panasonic), or Sony E-mount).

Other techniques you can consider doing without getting a new lens would be learning to use a flash, or learning how to ETTR (expose-to-the-right) and post-process for noise. A high ISO setting, by itself, doesn't actually increase noise as much as underexposure can. It could be that attempting to use a lower ISO setting is causing you to underexpose.

Also, with Canon cameras, the 1/3 EV ISO settings are done by digital push/pull processing. It's generally said that using the -1/3EV settings will result in less noise than the +1/3EV settings or whole stop settings, so using a multiple of 160 for your ISO setting (160, 320, 640, 1250), which is essentially doing ETTR in-camera, may help with noise.