Two Backend behind haproxy

We have two server behind haproxy one of this servers has resource issue and take time to do some process and send feedback to haproxy and haproxy answer request.
other server in backend is OK, and does not need make a lots of process.
all request go to haproxy and haproxy send some request to port 80 server 1 and send some request to port 80 another server.
server 1 need time to process some request and answer.
server 2 does not more time.
this week request to haproxy increase and sometimes we see error about socket timeout from server 1.
How I must config haproxy handle this problem?
I use default config of haproxy.