Well any "graphical improvement" patch will decrease framerate.

Is there any reason you need 250 fps? Does your monitor support over 90hz?

You want to target the framerate of your monitor.

Source engine games have asynchronous graphical and physical framerates, meaning your computer's physical framerate is not tied to your graphical frame rate, like in other game engines.

For CS:GO official servers have a 60 physical framerate, while most community servers opt for the maximum of 100. You will have more display latency improvement by reducing the Interpolate time than you ever will by getting framerates over 60fps. This is cl_interp_ratio and cl_interp_time. It goes by whichever is bigger. I recommend setting interp time to 50-75 for casual and 20-30 for hardcore. This is a substantial improvement from the 100ms of the default. Most gamers can "feel" the difference in the responsiveness of their shots.

Setting it to zero results in the use of extrapolation, which will result in errors and frequent error smoothing, which isn't good.