The primary node expects a reaction to some packet within a time frame (this time frame is adjustable by the --timeout option of drbdsetup). In case the timeout is not met by the other node the primary cuts the connection and tries to reestablish a connection.
In case the other node dies, your primary node will sit there and block all applications which are writing to the DRBD device. Basically it takes the time of the timeout until it decides that the other node is dead. Thus your applications may be blocked for this time.
This is caused when the IO subsystem of the secondary node is slow.
Increase the timeout. (Since connect-int and pint-int need to be greater than the timeout increase them as well)