If you can reduce the set of keywords you want to apply to 9 or less, you can solve this problem with keyword sets. You can use the keyboard shortcuts Alt/Option-Num to apply these keywords; they're arranged on screen so this is easiest to do with the numeric keypad.
The trick is, Lightroom is usually slow enough that you can "chord" several keywords before the smart collection removes the photo from the collection. I can apply about four keywords at a time this way. Once you get used to the physical arrangement of your keyword sets, you can do this without looking, in the same way that a musician doesn't have to watch his fingers when playing chords.
When I need to apply more than four keywords to a photo in a single operation or the number of possible keywords can't be reduced to a keyword set, I use Itai's solution: Cmd/Ctrl-K, then type all the keywords into the box, and hit Enter.
The chording solution works best in conjunction with many carefully-scoped smart collections that point out photos that need a certain class of keyword, along with a matching keyword set or two.
For example, I have a smart collection that warns me that I haven't given a photo any kind of location keyword. It can do this because my keyword hierarchy lets me search just for just a few high-level keywords: indoor, nature, studio... My "living room" keyword is actually "architecture > dwelling > house > indoor > living room" for example. Then once I have a photo tagged as indoor, I have another smart collection that checks for furnishing tags: chairs, carpet, windows, etc., since photos taken indoors rarely include no furnishings. Some of these smart collections have an "edit time is in the last day" type of rule so the rare photo that depicts exactly one subject doesn't remain in this collection forever.
In this way, I build up the keywords on my photos in layers, each pass potentially triggering further refinement.
That is to say, I solve your problem by purposely not attempting to do a Big Bang keyword fix to every photo.