After more research I found out in sencha forum the following information regarding the performance as the number of points increase: There's not a technical limit from the ExtJs framework as to how many points you can render in a chart / series. The performance of an ExtJs Chart is given by the points to draw by the engine. As the number of points increase, the performance will decrease. This is because the client (web browser) has to render the data locally. When the number of points is extremely high (e.g. 10000 points) most of the browsers will have problems in handle it, even just in memory. Moreover, in the above case, the chart should be around 3000px wide to actually display all the lines from point to point (wider than almost any monitor on the market).
In some cases a solution would be to thin out the data by applying some filter on the store. In my case, the number of points even applying time/date filters are so many that I don't get correct charts for ranges more than 15 days.