Code using single letter variable names and abbreviations makes people up to 15% slower at understanding code [1, 2].

Of course it is possible to determine - bottom up - what a variable is supposed to do or that p is short for pointer, it still slows others down because it requires them to perform mental mapping. This effect, of course, depends on experience. If you write your vars like this all the time your penalty won't be as big.

[1] Influence of identifier length and semantics on the comprehensibility of source code, https://github.com/cessor/bachelor-thesis

[2] Much shorter version published in https://fg-sre.gi.de/fileadmin/gliederungen/fg-sre/wsre2016/...