This wait event will not impact on user sessions. No end user has ever telephoned the help desk to complain that "the wait event such-and-such is too high". What are they complaining about? For example, is it a query that takes too long? Or a batch job that doesn't finish in time? You need to focus on the problem before trying to find a solution.
please suggest how to reduce the "ARCH wait on SENDREQ".
a) Speed up the network or
b) Change ARCHIVER to LOGWRITER for log shipping (recommended)
If the network is really slow and you have no options, you may choose to disable auto log shipping and do it manually