64 bit counter on BIG-IP 3900

The presence of 64 bit counters has nothing to do with the line speed. BigIP does support 64 bit interface counters, but in versions 11.2.0 - 11.6.x, the data is not under the ifMIB branch of the MIB, but is available at sysIfxStat.

Please refer to sol14473: The BIG-IP system now publishes network interface data through SNMP objects defined in the F5-BIGIP-SYSTEM-MIB file

In BigIP version 12.0.0 and later, both the ifXTable and sysIfxStat branches are populated with 64 bit counters, and contain data from the switching hardware. The 2000/4000, and virtual edition do not contain a hardware switch, so does not populate sysIfxStat