I recently came across a KB article (https://knowledge.broadcom.com/external/article/368812/how-to-change-the-vmxnet3-link-speed-of.html) that describes how to set the link speed of a vmxnet3 network adapter higher than the default 10 Gbps speed. In my lab I have several nested ESXi hosts and wanted to give this a try.
I powered down a nested host, edited the virtual machine settings, and added the advanced option ethernet0.linkspeed, setting its value to 25000 as pictured below.
After powering the virtual machine back on, I checked the ESXi network settings in vCenter and confirmed that the updated link speed was reflected, pictured below.
Expanding the vmnic0 details, I could see the adapter was still of type vmxnet3, and the link speed was correctly set to the specified value.
Conclusion
Updating virtual machine link speeds work for nested ESXi hosts as well. The valid range for linkspeed is 10000 to 65000. This makes common speeds, like 25 Gbps and 40 Gbps possible.