Hello,

Support for short-keys is added using a separate add-on:
https://addons.mozilla.org/en-US/firefox/addon/ctrl_groupspeeddial/
Multi-digits dialing can be then enabled in Options page / Short-keys.
The slow speed is most likely caused by some other add-on. There are known cases where this was due to "Ghostery" add-on or "Adblock Plus". You can try to use alternative add-ons like "uBlock" that should be faster and should not slow down other add-ons.

Displaying dials page should not take more than half a second even on the slowest PC!

Best regards,

Juraj Mäsiar