Number of widgets - at present Looker Studio will perform one query against Funnel for each widget in your report, even if they are displaying the same data (i.e. the same segmentation or breakdown) filtered in different ways. Minimizing the number of widgets therefore will have great impact on performance. Splitting your report into multiple pages might be a good place to start.

Transformation logic - having Looker Studio do a lot of data crunching will slow down your report, for example using Blends and Calculated Fields. Try to move as much of this as possible into Funnel and allow Looker Studio to focus on loading your graphs, not running calculations.

Cardinality - viewing your data by Date, Traffic Source, Market and Campaign compared to say Traffic Source and Market only will mean both Funnel and Looker Studio will have to process a lot more data. So, basically removing dimensions that aren't absolutely crucial can have good impacts on performance.

Filters - filters applied to your report are performed in Looker Studio rather than in Funnel. That means Funnel will have to load, process and transfer all data relevant for your query, and then let Looker Studio apply the filtering. This goes for widget specific as well as report wide filters. A common pitfall is for example a "Top 5 Campaigns by Impressions" widget, which will have to load data for every campaign from Funnel, before Looker Studio filters out the top 5.

Time period - essentially the same thing as a filter, but treated somewhat differently in both Funnel and Looker Studio. However the consequence is the same, if you have a report wide or widget specific period setting for a year, that means data for a full year will have to be loaded, processed and transferred. Limiting the number of widgets that displays time series, reducing the length of each time series, or displaying time series data for a summary rather each of your campaigns are things that can have a positive impact here.

Type of widget - analogous to the item above, using the right widget for your use case can be important. I.e. using a time series widget when you are interested in totals will cause a lot of extra data to be queries.