So, I don't know much about what is fast and what is slow in ZX BASIC, but here are a couple of small items to get this thread started. Hopefully some much more knowledgeable people will join in (I know you are out there, from browsing the entries - impressive stuff in there)!
1. Spectrum BASIC stores variables in the order you declare them, and when it needs to access one it searches linearly through them in memory. This means that variables that are going to be accessed most often should be declared first.
2. Shorter variable names also speed up the look-up, so use single character variables where possible.