Is there a bandwidth limitation on wired/wireless routers?

I read an article recently that though ISP sends data at higher bandwidth, the consumers will receive a lower bandwidth many times due to the limitation of the routers themselves.

I have seen that when I connect directly to my DSL modem, I can see a bandwidth of 8 MBPS and when i attach a wired router or VOIP gateway with router, I only receive 6 MBPS.

While surfing on web, I came across an article that said some routers/VOIP gateways limit the bandwidth.

My ISP’s DSL Modem shows a speed of 8.5 MBPS and when connected to a router and performed a speed test, it showed me 6 MBPS only