For a test, does a computer hardwired directly to the router / gateway device with an ethernet cable have the same problem ?
If you use only WiFi, the issue may be in the configuration of your WAP. 99.99% of the time, they come with a default configuration. That default also includes the connectivity speeds. Need to login to the WAP (Gateway Router if it is an all in one unit) and disable all speeds below 11mbps. You could even disable all 802.11bg (2.4GHz) speeds if you only have newer devices that connect. If you have any real old devices, you may need to keep the 11mbps (802.11bg) enabled. Otherwise force your devices to the higher speeds through the settings.