Opening a message with a long line is very slow due to coloring

Opening a message with a long line can be very slow due to coloring. On the following mbox example, it takes more than 10 seconds on my machine!

color-body-slow.mbox

The muttrc file to use:

color body lightyellow black /x+
color body lightyellow black [a-z]+@[a-z]+

Tested with Mutt 2.0.6+124 (using unrelated patches) and with Debian's package mutt 2.0.5-3.