If anything, writing "7x" is actually just a poor emulation of writing "7×".
Saying something is "done at 7x speed" should be read as "done at seven times speed" not as "done at seven x speed". So using the 'times' (multiplication) symbol is the better form in my opinion; it just happens to be significantly easier to type "x" instead, which is how we got here.
Saying something is "done at 7x speed" should be read as "done at seven times speed" not as "done at seven x speed". So using the 'times' (multiplication) symbol is the better form in my opinion; it just happens to be significantly easier to type "x" instead, which is how we got here.