readreg -> 5 seconds delay

Hello !

Using 'readreg' I get a 5 second delay before 'paste' is actually
processed, during which Screen displays:
"Slurped 3 characters into buffer".

Reproductible with the following sequence :
--
screen -S myS
# renaming window with Ctrl-A Shift-A myW
echo "ls" > .myB
# then from outside screen
screen -S myS -p myW -X readreg p .myB
screen -S myS -p myW -X paste p
--

I could use 'stuff' for the same effect with no such delay, but this
minimal exemple serves as an illustration of the problem I have with
Vim+Slime (1), which uses 'readreg' to avoid limitations on the number
of chars, I guess.

(1) https://github.com/jpalardy/vim-slime
On a Debian Sid/Jessie
Screen version 4.01.00devel (GNU) 2-May-06
GNU bash, version 4.2.45(1)-release (x86_64-pc-linux-gnu)