The limit for a 100mb port is 100mb. You have a bottleneck somewhere else. I have a seven year old PC with a built in 100mb NIC connected to a cellular router and it gets over 20mb up/down depending upon how busy the internet is. Late at night is usually the best.
I am talking practical real world speed vs internet speeds. They may call it a 100mb connection but it could be a gazillion and if I’m downloading a movie and it says it’s coming down at 12mb/sec as far as I’m concerned it’s 12mb.
In an ideal world, 1,000 Mbps = 125 MB/sec
“Mbps” = Megabits per second
Eight bits = one byte
“MB/sec” = Megabytes per second
In reality, it may be that 10 bits = 1 byte, due to error correction overhead per byte delivered. Add in plenty of additional overhead for management and loss and your 1,000 Mbps connection might get you 90 MB/sec.
If we are talking about a “100 mbps” connection, that would be 9 MB/sec, practical maximum.
Scale up or down for added speed, so a 600 mbps connection would be 54 MB/sec.
Now, add in the losses from WiFi, in which the bands are often harassed by neighbors using the same frequencies, you can conceivably cut your “sped” down by a chunk more.
Then, add in that your range extender's physical port is 100mbps, and you have a recipe for the problem you are noting.