Are you using charts? If so then work out how many points you are plotting on each line on the chart. So for example is a chart is configured with a range of 24 hours and you are adding points every 5 minutes that would be 288 points. However if you were adding a point every 5 seconds then that would be 17280 points, and if you had 4 charts each with 4 lines that would be a total of 276480 points to be shuffled down every 5 seconds. This can have a dramtic effect on the system, particularly the browser. Note that there is little point having more that one data point for each pixel on screen, so for example if your graph were 200 pixels across with a range of 24 hours then the max sensible sample rate is about every 5 or 10 minutes.
If you aren't using charts then tell us a bit more about the system. What hardware/os, how did you install node-red. Are you running the browser on the node-red server machine or on a separate device? If you run a system monitor (such as
top on a pi) does it show anything notable?