Slow down with two devices

My fiance works from home and is a digital asset manager. In a nutshell she's downloading and uploading images during the day. Her computer was on wifi the past 4 years when I noticed, and recommended she hardwire it into the modem. We are on the 500 Mbps plan. Today I'm off work and wanted to do some online gaming, but she says it would slow her down. She says I can stream on the TV though. Is there any logic to this? Does our plan not allow for two devices on at one time? Would online gaming seriously affect her ability to perform her daily work?