This does sound strange, and it's not something I've heard before. I would have to think that either there is some difference in the SNMP configuration (views applied, ACL, etc.) or perhaps the problematic string contains a '@' and the switch is evaluating it with community string indexing rules.
Before any kind of troubleshooting can begin, you'd need to get the show ver and show run. It may also be helpful to get a sniffer trace just to see if anything on the network might be throttling SNMP responses based on community string (would be strange, but you never know). Seeing the switch's CPU utilization at each poll would be helpful, too. Perhaps it's coincidental that one string appears to take longer than another. Perhaps something else is happening on the switch that is driving up CPU utilization at the same time.
