Download speed: wired vs. wireless

Hello - I just replaced my Zoom modem/Apple wifi router setup with Xfinity Gateway modem/router. My Comcast plan has 600mb DL, after the easy install and activation my wired DL is under 100mb while my wireless DL is over 500mb. My PC is an older Dell Inspiron 64bit dual core, 3GB RAM. Spent a long time with tech support chat agent but unable to help.

Any idea why wired PC DL speeds would remain slow? These wired sppeds are what I had before equipment upgrade.

thanks,