Reducing 'max bars on chart' or 'max bars in history, will affect on your EA's performance, if it makes calculations depending on the charts. For example if it calculates the trend by looking last two years and you reduce the bars- of course this will affect the EA. But if the EA doesn't use similar calculations / or places random orders - I think it will be not a problem to reduce the bars. Shortly- depends from your EA's working model
I think there are better ways to improve the PC's performance. For example, If you are using an automatic EA, you can remove all your indicators from the chart. The EA no needs indicators to be placed on the chart. It gets the needed from the database. No need to place any indicators on the chart in order to work the EA. I mention the indicators, because some indicators takes a lot of RAM.
Or if you are using an automatic EA, you can Zoom In your chart- this will save from your RAM, without affecting your EAs working performance.
Finally, the best way to improve the PC performance is to add additional RAM or buy more powerful PC I know the RAM is the best key for graphic and working performance of your terminal + strong Internet connection for faster executions.
Regards