If you have a lot of metrics on a single dashboard, it can really slow down the platform as it tries to load the data and render the charts for each metric. In general, we recommend that you have 20-25 metrics on a single dashboard.
Table charts and Pie charts can take the most time to render, so if you have several table charts (especially if they have many rows) or pie charts (especially if there are over 30 slices) on a single dashboard, it can start to affect performance around 10-15 metrics.
Try to think of ways you might be able to simplify some of the charts or reduce the date ranges visible for each chart. If possible, separate some of the charts onto their own, more focused, dashboards.