Given you are mentioning 100Mbps wireless dongle I'll assume it is using the Wireless N (802.11n) standard rather than Wireless G (802.11g) standard. However, if you are using a Wireless N dongle to connect to a Wireless G Router it can and will fall back and use Wireless G speeds.
Another thing that may be confusing you is bandwidth vs data rates. Bandwidth is typically measured in BITS (small b) per second such as Mbps while data is measured in BYTES (big B) such as MB/s. A bit is 8 times smaller than a byte. When I need to convert between units I use this tool here: http://web.forret.com/tools/bandwidth.asp
Wireless G = 54 Mbps = 6.75 MB/s
Wireless N = 100 Mbps = 12.5 MB/s
USB1.0 = 12Mbps = 1.5MB/s
USB2.0 = 480Mbps = 60MB/s
Other things to take into consideration.
1) Radio (Wireless) medium is subject to interference. When there is interference speeds go down due to things needing to be retransmitted
2) Radio is also a SHARED medium. Thus if your RPi is connected, your phone, your laptop, your XBox/PS3, etc... each of those is taking a bite out of your bandwidth as well.
3) 2.4 GHz band is heavily populated it includes other peoples wifi, cordless phones, baby monitors, intercoms, etc... all use these frequencies.
Thus before you go out and buy a new dongle. Keep in mind that new dongles might not solve your problems. Especially if it's a USB 2.0 Wireless N Dongle connecting to a Wireless G router.
Also dont forget Ethernet overhead takes up some of the potential as well. Thus even with a 100Mbps connection it wont reach it's full 12.5MB/s.