I believe the F5 support team usually recommends querying not more frequently than every 10 seconds (see SOL6644 on support.f5.com). Also, the snmp service caches responses for a given table for 2 seconds, so you won't see any new information querying every second vs. every 2 seconds. If you just want live data (not for archival), try the "bigtop" command on the CLI (see "bigtop -h" for syntax).
As for the penalty, it really depends on how large your configuration is. When you query anything from one table, I believe the snmp agent internally fetches the data for that entire table (I'm not certain, you'd have to ask support). So if you have a large configuration (many virtual servers for example), your simple query might internally require fetching thousands of data points. You can monitor the impact by watching the CPU usage of snmpd and mcpd while you're doing queries every 2 seconds. Anecdotally, it's been my experience that if you have only tens of virtual servers you can usually get away with a lot of monitoring without any real impact.